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These observations explain how it is that blankets will not bear boiling in water; by the removal of the grease and felting together of the wool they lose their softness and warmth, and become hard and threadbare. They may be washed in cold water or exposed to dry heat of moderate temperature without much deterioration, but a frequent repetition of these processes brings about in time a change similar to that effected by boiling water. Moreover, if wool be piled together when hot, the hairs, through the softening of the coatings of grease surrounding them, become glued together so that when cold the wool is felted into masses.

(247) Wetting.

(7) Wetting is undesirable in the case of some kinds of goods for it produces shrinkage and causes the colours to run and involves. the labour of subsequent drying. We need only consider wetting in connection with steam. When a cool body, such as a pillow, is placed in an atmosphere of steam, some of the steam will be condensed upon it, and if the relative volume of the steam be sufficient, the temperature of the cool body will be raised to 212° F., and this temperature having been attained no further condensation of steam into water will take place. Owing, however, to the high specific and latent heat of steam, the weight of steam thus condensed will be much less than the weight of the body heated to 212° F., by its condensation.

In the experiments with Benham's apparatus, in which the steam is not superheated, the increase of weight in a pillow exposed to the action of the steam was found in two experiments to be 13 and 14 per cent. respectively, of the previous weight of the pillows. On the other hand when one of the pillows was placed under a tap and saturated with cold water, it was found to have increased in weight 140 per cent. The amount of water derived from the condensed steam was therefore only one-tenth of the quantity which the pillow would hold when saturated. A roll of flannel in the same machine increased in weight 22 per cent., a coat 36 per cent. The amount of moisture absorbed depends not only on the nature of the materials, but upon their previous state of dryness or dampness; if wet, extra heat would be required to

raise them to the temperature of the steam, and more steam would thereby be condensed.

In Lyon's apparatus, in which the steam is used at a higher temperature, being superheated by the higher degree of heat corresponding to the extra pressure on the outer case, the amount of wetting is less; in experiments with bales of rags the increase of weight varied from 3.5 to 5 per cent., the greater part of which was again lost on a few hours' exposure to air.

(248) The different Forms of Apparatus for Disinfection by Heat.

In Dr. Parson's paper, already so largely quoted from, he considers that the most important requisites of a disinfecting chamber are the following: (a) uniform distribution of heat in the interior ; (b) a constant temperature maintained during disinfection; (c) facilities for ascertaining the actual temperature of the interior at any given moment.

In apparatus heated by steam, the three requirements are satisfactorily met, and in some of the best dry heat chambers the results are also fair, but in most of them condition (a) is not fulfilled.

As a type of a steam apparatus Lyon's will be described; as types of hot air apparatus, Bradford's, which is heated by coal, and the Nottingham self-regulating apparatus, heated by gas.

(249) Washington Lyon's Patent Steam Disinfector.

The machine consists of a large strong iron chamber with double walls of boiler plate, and a tightly-fitting door at either end, the door shuts against an india-rubber collar, and is secured by vice handles so as to form a steam-tight joint; it is hung on hinges and its weight when swung open is borne by a castor running on a curved rail. By means of a steam pipe, steam from a boiler can be admitted into either the hollow casing or the interior of the chamber; another pipe serves to let off the steam when no longer required, and there is also a means of escape for condensed water which may accumulate in the casing. The casing and chamber are each provided with a safety valve and pressure guage, by which the pressure of steam, and hence the temperature, can at any moment be seen,

the degrees of temperature being marked on a dial and indicated by a needle.

In using the apparatus steam is first turned on so as to heat the casing or jacket, and in this way the walls are warmed up. The articles to be disinfected are then put into the chamber and the doors closed. It is well to allow the steam to circulate in the outer casing some time, so that the articles will become quite hot, for in this way when steam is admitted into the chamber itself, there will be little condensation or wetting. The steam is admitted ultimately into the interior of the chamber, until the gauge marks the desired temperature. On the conclusion of the disinfection, the steam is shut off from the interior, but allowed to circulate in the casing. On opening the doors, in a very little time the articles will be quite dry.

(250) Bradford's Patent Disinfecting Apparatus.

It essentially consists of two parts, the one a container, the other the heating apparatus. The container is a large rectangular iron box, covered with a non-conducting composition, open below and suspended by chains and counterpoises from pillars something like a gas-holder. The roof has a ventilating aperture, a thermometer is at the side, the bulb projecting into the interior, the stem being placed outside.

The heating apparatus consists of three longitudinal compartments side by side, the central one containing the fire waggon. The fuel is preferably peat, but coal, coke, or charcoal may be used; the fire is in a waggon which may be drawn in or out. The firechamber containing this waggon has a roof of hollow iron bars, in section triangular, open at the ends. The fire chamber has a flue at one end, a door at the other. The two side chambers are for purposes of ventilation and communicate with the fire chamber by means of side valves.

The articles to be disinfected are placed on a galvanized iron rack which stands over the fire chamber.

The container resting on a layer of sand, a large shallow vessel containing water is placed at the bottom over the fire chamber so as to keep the air of the chamber moist.

The whole apparatus may run on wheels so that it can be moved from place to place.

(251) The Nottingham Self-Regulating Disinfecting Apparatus.

This is the invention of Dr. Ransome, F.R.S. It consists of a cubical iron chamber cased in wood, with an intervening layer of felt, access to the interior being obtained by double doors. The furnace, placed at the side of the chamber and on a lower level, consists of a ring of atmospheric gas burners, enclosed in an iron tube. The heated air, containing the products of combustion, passes along a horizontal flue and enters the chamber at the bottom, which is perforated by a number of holes for its equable distribution. In the horizontal flue are fixed the bulbs of a thermometer and of a self-acting mercurial regulator. Through the latter the gas supply to the burners can be made to pass, and it is so constructed that as the temperature of the apparatus rises the mercury expanding encroaches upon a slit, through which the gas passes, and thus gradually cuts off the supply. At the top of the chamber there is an outlet flue, controlled by a valve and furnished with a thermometer. In connection with the outlet is an arrangement designed for the extinction of fire; when the temperature at the outlet exceeds 300° F. a link of fusible metal melts, closing a damper and shutting the supply of gas. The chamber is fitted with bars and hooks for suspending articles of clothing. When the stove is first lighted the gas is admitted to the burners direct through a short circuit pipe, without passing through the regulator, but when the mercury in the latter has risen high enough to reach the slit, this pipe is closed by a trap so as to compel the gas to pass through the regulator. The regulator is furnished with an adjusting screw, so that it can be set to work at a higher or lower temperature as required.

CHAPTER XXIV.

CHEMICAL DISINFECTANTS.

(252) List of Practical Disinfectants.

RESTRICTING as before the term disinfectant to a substance which is capable by its own inherent poisonous action upon a pathogenic micro-organism to destroy its life and therefore prevent its development, the list of practical disinfectants is a small one; all disinfectants that are expensive must be thrown out, all those which do not admit of being applied by reason of their insolubility in water, or from other causes must also be excluded, hence in this work the following disinfectants will alone be treated of at any length :-Corrosive sublimate, the halogens, iodine trichloride, the tar acids, sulphur dioxide. To this list might be added aniline on account of its remarkable disinfecting action upon the tubercle bacillus, but there is no likelihood of it becoming a disinfectant in general use. The salts of zinc, of manganese, of lead, of iron, the mineral acids, and a number of mixtures vaunted by their proprietors, may be shown generally to be of doubtful value, save in large doses, and under conditions which are only occasionally met with in practice.

(253) Mercuric Chloride. Corrosive Sublimate. Bichloride of Mercury, HgCl2

This well-known highly poisonous salt is in the form of a heavy crystalline powder or crystalline masses. A saturated aqueous solution contains about 10 per cent., but two water dissolve one part of corrosive sublimate. soluble in alcohol or ether.

parts of boiling It is also readily

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