change. Parsons's experiments are more definite, because the tensile strength was tested. The results are contained in the following table:

TABLE XXXV.

DR. PARSONS'S EXPERIMENTS ON EFFECTS OF HEAT ON VARIOUS FABRICS.

Scorching is especially liable to take place when the heat is in the form of radiant heat. In one of the experiments with different forms of apparatus, it was found that the ticking of a pillow was uninjured, although exposed to heat of 276° F. as tested by a thermometer laid beside it. In this case the pillow had been screened from the direct access of heat rays from the source of heat, whereas in other instances, in which the pillows were laid on wooden bars directly over the heated bottom of the chamber, it showed distinct stripes of scorching corresponding to the interspaces between the bars, although a thermometer laid beside it did not record a temperature any higher than, or even so high as, in the former case.

Scorching is brought about by radiant heat in two ways-First, textile articles, specially wool, are good radiators, as we may see by the copious deposit of dew or hoar frost which takes place upon them when exposed in the open air on a clear night, and therefore with converse facility they absorb the rays of heat, and are thus heated to a higher degree than the surrounding air. Secondly, in the chamber heated primarily by radiation the walls of the chamber and solid objects in its interior, such as shelves, bars, and hooks, become by absorption of heat-rays hotter than the air, and consequently organic substances, such as articles of dress, are liable to be burnt where they may chance to come in contact with the hot metal. On the other hand, if the chamber be heated by hot air warmed before its entrance, as in the Ransome stove, the walls of the chamber are no hotter than the air, and objects may come in contact with them without injury.

(243) Over-drying.

(2) Over-drying, short of actual scorching, renders many organic substances brittle; it is by this means that in pharmacy tough vegetable drugs, as roots, barks, and others, are prepared for grinding into powder. On the other hand, on exposure to moist air this brittleness is again lost; we know how the crispness of the new-baked biscuit is soon lost unless kept in a well-closed canister.

It was found by Dr. Henry, in his experiments, that the tensile strength of textile materials was weakened by exposure to dry

heat, but was in a great measure regained if the materials were allowed to remain in the air a day or two before testing.

M. Vallin1 took hair and wool, which had been previously well beaten, and exposed them for four hours to a temperature of 248° F. Portions beaten again immediately after being taken out of the stove yielded a layer of detritus, while other portions, beaten only after a lapse of twenty-four or forty-eight hours, when they had time to regain their hygroscopic moisture, did not yield more waste than unheated portions of the original material.

It is evident that injury from over-drying can easily be avoided by allowing the materials which have been subjected to dry heat to remain in the air long enough to recover their natural degree of moisture before manipulating them.

(244) The Fixing of Stains.

(3) An exposure to heat of 212°, whether in the form of dry heat, of boiling water, or of steam, has the effect of fixing many organic colouring matters in animal and vegetable fibre, so that they cannot be removed by subsequent washing; in fact high pressure steam is used by dyers and calico printers in an apparatus called a "keir," for the very purpose of fixing their colours. This fixing of stains is specially marked in the case of albuminous materials coagulable by heat, such as blood. It is well known that in order to remove blood stains the cloth or garment must be steeped in cold water.

The following experiments were made on this point:-Pieces of linen sheeting, marked distinctively, were steeped at one end in fresh sheep's blood, in dirty animal grease, and in claret, strips of calico were also steeped in ox-gall and in urine, and after exposure to heat the strips were washed in Messrs. Lyons's establishment in the ordinary manner with boiling water and soap and soda. The results are in the table on the following page.

This property of heat is an inconvenient one from our present point of view, and seriously limits the field for practical usefulness of disinfection by heat. It is very desirable that linen articles, sheeting, body linen, and such matters, which have been in con

1 Op. cit.

tact with infection, should be disinfected before coming into the laundry, since otherwise they may infect the washerwomen and possibly the linen of other households. Such objects however, if soiled with blood and fæcal matter, cannot be disinfected by heat, not even by boiling water, without indelibly fixing the stains. The only alternatives therefore are to put up with damage to this extent, to allow the articles to pass through the earlier processes in an undisinfected state, or to attempt their disinfection by chemical means. When the grosser dirt has been removed, by soaking and rubbing in cold or tepid water, the articles may be boiled without injury, and are then doubtless effectually disinfected.

TABLE XXXVI.

Results after treatment and subsequent washing with soap and water.

(245) Melting of Fusible Substances, such as Wax and Varnish.

(4) Injury may in some instances be caused by the melting of wax and such like fusible materials in ornaments or contained in the pockets of garments. Articles occasionally catch fire when the heat of the apparatus does not exceed the normal degree, doubtless owing to lucifer matches being left in the pockets.

It is sometimes necessary to disinfect articles of furniture with stuffed seats, such as chairs and sofas. If the heat be too intense the varnished wood-work will blister, and in steam the softening of the glue will loosen the veneer.

(246) Alterations in Colour, and Shrinking.

(5) Dry heat short of actual scorching does not often affect the colours of dyed goods. If not properly fixed or, in technical language, "fast," steam or boiling water will cause them to run.

(6) Dry heat causes little shrinkage in woven materials. Moist heat, on the other hand, or even wetting without much heat, causes permanent shrinkage, more especially in the case of woollen goods, as cloth, flannel, and blankets. The cloth of which men's clothes are made is, "or should be," shrunk before they are made up; this shrinking is effected by exposing the cloth to steam. It will be seen however from the preceding Table that the amount of shrinkage of flannel was quite as great after simple washing as after exposure to steam. A piece of washed flannel 50 inches long (measured after washing) only shrunk inch further, or per cent., on exposure for half an hour to steam at 228°.

A more serious drawback is the felting together and loss of elasticity and fluffiness, upon which the warmth and softness of woollen materials depends. This elasticity depends upon the presence of the natural grease of the wool. M. Leblanc (quoted by Vallin) states that this grease (suint), which is a product of the perspiration of the sheep, is a mixture of mineral salts and organic compounds; in unwashed wool it amounts to nearly 50 per cent. of the total weight of the wool, and even in washed wool never falls below 15 per cent. This grease is of a nature highly putrescible under the combined influence of moisture and heat; so that the quantity of it contained in a mattress, together with the contaminating infiltrations from human bodies and the excrements from moth larvæ (amounting in bedding often to 1 per cent. of the whole) form collectively a very large mass of decomposing matter, which must add considerably to the contamination of the air of inhabited dwellings. On the other hand, if the wool is too well cleaned, so as to entirely remove the natural grease, it loses its strength, suppleness, and elasticity, the qualities for which it is specially valued. The suint is soluble in boiling water: water in which wool has been boiled for an hour is charged with an extremely fetid matter, rich in sulphur products; after this treatment the wool resembles cotton, it easily felts, and has permanently lost its elasticity.