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Apply with copies of testimonials and stating qualifications to the Secretary, The Oxford Portland Cement Co., Ltd, Kirtlington, Oxford.

FOR SALE.-Bunge Balance, cost £10,

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GEORGE HERIOT'S TRUST.

HERIOT-WATT COLLEGE, EDINBURGH.

'he Governors are about to appont

The

a

PROFESSOR OF CHEMISTRY to devote his whole time to the work of this Department of the College, but he may be allowed, under certain conditions with the consent of the Governors, to undertake consulting and commercial work.

In making the appointment the Governors will give preference to thr applicant who has both teaching and industrial experience. By the term "industrial experience" is meant that the applicant has had special knowledge of some Department of Industrial Chemistry, either by being employed in connection with some process of chemical manufacture, or through a consulting practice has had experience of manufacturing processes.

The salary will be not less than £600 per annum.

Further particulars may be had on application to Principal LAUR.E at the College, or to the undersigned.

Applications, with 30 Copes of Testimonials, to be lodged with the undersigned on or before JUNE 27th.

PETER MACNAUGHTON, S.S.C.,
Clerk to the Governors.

CHEMISTRY COURSES.

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FOR the CONJOINT BOARD & FIRST MEDICAL EXAMINATIONS.
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THE ELEMENTS OF CHEMISTRY.

By H. LL. BASSETT, B.A., B.Sc., Demonstrator in the University Chemical Laboratory, Cambridge. With an INTRODUCTION by Professor W. J. POPE, M.A., F.R S., Professor of Chemistry at Cambridge University.

This Text-book is specially designed to meet
the Requirements of the Conjoint Board,
First Medical, and other Examinations.

CHEMICAL NEWS, June 5, 1914

}

SINCE March, 1909-in connection with the Glass Workers' Cataract Committee of the Royal Society-I have been experimenting on the effect of adding various metallic oxides to the constituents of glass in order to cut off the invisible rays at the ultra violet and the infra-red ends of the spectrum. The work has been done chiefly in my own Laboratory. I have been aided by Mr. Harry Powell, of the Whitefriars Glass Works, who prepared several pots of coloured glass from my formulæ on a much larger scale than could be made outside a Glass works. From these glasses cylinders and sheets were made.

The main object of this research is to prepare a glass which will cut off those rays from highly heated molten glass which damage the eyes of workmen, without obscuring too much light or materially affecting the colours of objects seen through the glass when fashioned into spectacles, but the work necessitated an examination of the screening properties of glass plates for ultra-violet and luminous light, and therefore the research was enlarged so as to embrace the three forms of radiation.

Radiation from Molten Glass.

In order to ascertain what rays are given off from molten glass I spent some time at the Glass Bottle Works of Messrs. Nuttall and Co., St. Helens, and took many photographs of the spectra of the radiations.

Photo-spectrographic and other examinations were made of the radiation emitted from the molten glass under working conditions. Full details of the experiments and results are given in this paper.

At the time I visited Messrs. Nuttall's Works light green bottle-glass was being made; the mixture is composed of silica, sodium sulphate, and calcium carbonate or sulphate. The materials are melted in a large firebrick tank, heated by a flaming mixture of gas and air playing on the surface. The gas is made some distance from the furnace in a "producer." Gas and air are conducted by separate channels to the upper part of the tank, where they mix and burn, the flame reverberating from the arched roof and heating the glass mixture to the requisite degree.

The area of the tank of molten glass is about 82 square yards, and it contains from 300 to 350 tons of the mixture. There are several such tanks in the works. The tank is divided by a fire-clay partition into two unequal parts. At the lower part is an opening through which the melted glass can flow. The larger portion of the tank, where the materials are melted together at a high heat, has a surface of about 63 square yards. This is called the "melting end"; when the mixture is well fused and homogeneous the molten glass flows through the opening into the "working end" of about 19 square yards, where the heat is less and the glass is in a viscous state. Fire-clay rings of 18 inches internal diameter and a foot deep float on the surface of the viscid glass; any scum on the surface of the tank is thereby kept from contaminating the surface of the glass inside the ring. One ring floats opposite each working opening, and the workmen with draw the requisite quantity of glass for each operation from the inner surface of the ring.

* Read before the Royal Society, November 13, 1913. From the Philosophical Transactions of the Royal Society, Series A, vol. ccxiv., pp. 1-25.

265

The light from the melting end of the tank, viewed through a working opening, was brilliant white with a tinge of orange; it was with difficulty the unprotected eye could make out any details. Viewed through dark glasses the surface of the metal in the tank appeared as a seething mass in constant commotion. The surface in the working end was more easy to see. It was of a bright yellow incandescence, and comparatively quiet.

It is not certain what the temperatures are at each end of the tank. So far as one could judge the temperature at the melting end is about 1500° C., and at the working end decidedly less say, 1200°.

About each opening, especially at the melting end, thin white vapours rose and settled on the surrounding cooler parts. A piece of paper held in this vapour instantly ignited. Examined with a hand spectroscope the yellow line of sodium was seen to be brilliant in this vapour, but the light from the molten glass showed a continuous spectrum in which the sodium line was visible. On one or two occasions a black line was seen in place of the yellow sodium line, showing a reversal. Some of the condensed vapour was collected from the cool sides of the working opening and chemically examined. It was found to consist principally of sodium and calcium sulphates, with a little sodium chloride.

Photo- and Thermo-graphic Experiments.

The spectrograph used for taking photographs of the radiation from the molten glass is the one I described in Roy. Soc. Proc., vol. lxv., p. 237, May, 1899. It has two quartz prisms, each made up of two halves, one half being right- and the other half left-handed, according to Cornu's plan for neutralising the effect of double refraction. The collimating and camera lenses and the double condensers are also of quartz cut in the same fashion. The slit jaws are made of two acute angled quartz wedges, edge to edge. The refracting prisms are of 60° angle, and each face is 35 mm. by 42 mm. The lenses are 52 mm. diameter and 350 mm. focus. The condensers are plano-cylindrical, one being double the focus of the other. In order to ascertain the exact position of any part of the spectrum I might obtain from the radiation from the molten glass, I took photographs on each plate of an alloy of equal molecular weights of zinc, cadınium, tin, and mercury. This alloy gives throughout the photographic region lines, the wavelengths of which are well known.

The instrument sloped downwards, so as to allow the radiation from the surface of the melted glass to enter the condensers, prisms, and slit along the axis. To prevent the great heat injuring the spectrograph Mr. Nuttall allowed the opening to be bricked up, leaving a hole a few inches square in the middle. This was covered with an iron plate with a 2-inch hole in it, and over this a quartz plate was fixed.

Panchromatic films were used. These are sensitive beyond à 7800 in the ultra-red, and to the highest ultraviolet rays which will pass through quartz (about λ 2100). Flexible films had to be used in preference to glass, as they had to follow the curvature of the focal plane. Many preliminary experiments were made to ascertain the extent of spectrum to be recorded, its best position on the films, and the exposures needed. The slit of the instrument was generally placed about 4 feet from the molten surface, and it was found that from ten to fifteen minutes were required to produce a faint image on development. On each film, immediately before the radiation picture was taken, a photograph of the spark spectrum of the quadruple alloy was impressed on the film, in such a position that the two spectra would overlap to a very slight degree.

No. I photograph was taken at the working end of the tank, where the temperature was lower than at the other end. An exposure of twenty minutes was given, the width of slit being o'025 mm.

No. 2 photograph was taken in the same conditions as No. 1, but with an exposure of forty-five minutes.

No. 3 photograph was taken at the melting end, where

for three hours.

It was not found practicable to give longer exposures. Whilst these experiments were in progress, other experiments at another opening at the hottest end were tried to see if X-rays could be detected. Sensitive films were wrapped in black paper and then in lead foil in which designs had been cut. These were exposed for varying lengths of time, as near as it was safe to put them to the radiation from the molten glass, bearing in mind that the heat might affect the films. On development, no image of the stencil designs on any of the films could be detected. These results confirm those previously obtained by Dr. Burch-that X-rays are not emitted by the highly incandescent molten glass.

A careful examination of the six photographs shows a general progressive character, the extent of spectrum photographed extending into the ultra-violet as the length of exposure increases.

No. I photograph, exposed twenty minutes, extends to λ 4520.

No. 2 photograph, exposed forty-five minutes, extends

No. 4 photograph, exposed sixty minutes, extends to λ 3640.

No. 5 photograph, exposed 120 minutes, extends to ▲ 3595.

No. 6 photograph, exposed 180 minutes, extends to λ 3345.

Taking the ordinary limit of visibility to lie between A 3900 and A 7600, it is seen that with an exposure of three hours to the highest heats the strength of impression

does not extend much into the ultra-violet.

The heat

rays are very strong, and if injury to the eye is caused by exposure to radiation from the molten glass, a protective glass should be opaque to the infra-red rays.

These being present in the radiation from molten glass in far greater abundance than the ultra-violet rays, the inference is that it is to the heat rays rather than to the ultra-violet rays that glass workers' cataract is to be ascribed. It is, however, certain that exposure to excess of ultra-violet light also injuriously affects the eye. That the ultra-violet rays act on the deeper-seated portions of the eye is shown by the intense fluorescence of

the crystalline lens induced by these rays.

Besides the invisible rays at each end of the spectrum, the purely luminous rays, if present in abnormal intensity, are found to damage the eye. It therefore would be an advantage if in addition the obscuring glass for the spectacles were to be of a neutral or grey tint.

Synthetic Preparation of Glasses.

It soon became evident that my best, if not only, chance of solving the problem was to make different glasses in my own Laboratory, with the addition of known quantities of pure metallic oxides and earths as colouring or absorbing materials. Lapidary apparatus for cutting, grinding, and • In connection with this table the following scale for correlating

colours with wave-lengths will be useful:Wave lengths 7230 and below =

እ

Infra-red. እ

From 5920 to 4550 = Blue. ", 4240 = Indigo. = Violet. " 3970 3970 and above = Ultra-violet

6470 5850 Orange.

5850,, 5750 = Yellow.

39

4550 4240

5750, 4920 = Green.

stirred at frequent intervals with a stout platinum rod. After an hour the stirring is discontinued, and the temperature kept up for an hour and a-half. The current is then cut off, the openings in the furnace plugged with asbestos to prevent draughts, and the whole allowed slowly to cool to anneal the glass. In some cases the composition of the glass was such that the melting-point had to be raised above 1400° C., and as this temperature was beyond the safe limit with the platinum strip furnace, a blast-furnace fitted with a "Lennox electric blower was used; with this arrangement larger quantities of glass could be raised with safety to a much higher temperature.

There are two conditions I have endeavoured to secure of the finished glass-each of great importance. One, the most essential, is the absence of all streaks, striæ, and irregularities of density; the other, the absence of air bubbles. The first is obtained by repeated stirring and perfect admixture; the freedom from air-bubbles is secured by leaving the glass in perfect repose while the heat is at the highest point. On these and other points I have been much aided by reading an early paper by Faraday "On the Manufacture of Glass for Optical Purposes," the Bakerian Lecture read before the Royal Society in 1829 (Phil. Trans. Roy. Soc., 1830, p. 1). On a small scale it is almost impossible to avoid slight striæ owing to dif ferences of density caused by the long continued beat volatilising some of the soda. Faraday was much harassed by this dilemma in the manufacture of his optical glass, and tried many experiments to ascertain the cause. To get rid of air-bubbles Faraday used spongy platinum in powder sprinkled over or added to the bulk of the melted glass. This was found to act pretty well, making the bubbles rise in the same manner as a piece of bread causes bubbles to rise when thrown into a glass of effervescing liquid. To get the full benefit from this device, however, the glass must be kept perfectly quiet and at the highest temperature for a longer time than in my case was always practicable.

When the crucible has cooled slowly for about twelve hours it is removed from the furnace, and the solid cone of glass removed by breaking the crucible by a few judicious blows with a light hammer. The lump of glass is

now taken to the lapidary's table, and slit across the middle and a plate cut from it, which is ground and polished to a determined thickness, usually 2 millimetres.

Testing Synthetic Glasses for Opacity to Ultra-violet

Light.

The plates are now tested for the opacity of the glass to the rays of the ultra-violet end of the spectrum. The