I IK AT. 



411 



Tbo addition of salt* to water lower* it* freezing point, benoe 

 sea-water doea not freeao till 4 or 5 below 32" ; wbeo, bow. 

 the salt* are loft in tbe water around, 

 iiinl tlio ioo is perfectly pore. If all ti. 



in then allowed to oool withmr 



ih.-tnrli, !, :ii. i : . ;med to a low teuperaturo, it will not 

 freeze till several degree* below 32 ; but M soon M any ioe 

 fornu, tbo rest of the water will at onue rue to tbat point, 

 clearly showing tbat the latent beat of water in giren off M it 

 freezes. Thia fact clearly explains why a coat of ioo form* to 

 slowly. Woro it not for thin provision, M soon M any maM of 

 water had sunk to the temperature of 82, it would beoome a 

 maas of ico ; but now, every particle M it freezes give* out iU 

 heat, and thus raises the temperature of the rest. 



In the same way, were it not for the latent heat of steam, 

 the moment when water attained the boiling point would be a 

 rery dangerous one, for it would then be immediately converted 

 into steam with an explosive force greater than that of gun- 

 powder, since a cubic inch of water occupies nearly a cubic foot 

 when converted into steam. 



Dissolving any substance in a liquid always lowers the tem- 

 perature. Thia may easily be seen by throwing some soluble 

 salt into water, and can-fully observing tho effect produced on 

 a thermometer placed in it. Thia absorption of heat during 

 liquefaction is turned to account in the preparation of freezing 

 mixtures for tho production of artificial cold. In these, two 

 or more substances, which have a chemical affinity for each 

 other, and of which one, at least, is a solid, aro mixed together, 

 and during tho solution a considerable amount of heat is ren- 

 dered latent. 



Many different mixtures have been used, one or two of which 

 we give hero. 



A mixture of about two parts of enow or pounded ioe to one 

 of salt will reduce tho temperature to O p . This point, in fact, 

 was chosen by Fahrenheit as the zero of his scale, as he believed 

 it was the lowest temperature attainable. The mixture rapidly 

 liquefies, and if a small vessel of water bo placed in it, tho 

 water will speedily bo frozen. A mixture of C parts of sulphate 

 of soda, 5 of nitrate of ammonia, and 4 of dilute nitric acid, 

 will cause a still greater reduction of temperature. 



As wo have seen, water, on attaining tho temperature of 212, 

 enters into a state of ebullition ; a large number of bubbles of 

 steam aro produced at the part of the vessel which is exposed 

 to the source of heat ; these rise through the liquid, violently 

 agitating it as they burst. The point at which this ebullition 

 commences is that at which the tension of tho steam becomes 

 sufficient to overcome tho pressure of tho atmosphere ; and 

 hence, if this pressure be increased, the boiling point will bo 

 raised. 



Thus, though the boiling point of water is said to be 212, 

 this is only true when the barometer stands at 30 inches ; when 

 it is lower than this, water boils at a lower temperature. 



In an open vessel the temperature of a liquid can never be 

 raised above its boiling point, as all the surplus heat received 

 is employed in evaporating the water. If, however, a closed 

 vessel bo employed, the pressure may be increased, and a much 

 higher temperature attained. The apparatus usually employed 

 for this purpose is known as Papin's Digester, and is repre- 

 sented in Fig. 14. It consists of a strong iron vessel, D, the 

 lid of which is fixed on tightly by means of a screw, B. A safety- 

 valve, 8, is also provided, so as to allow of the escape of the 

 vapour when its elastic force becomes too great. In this way 

 a temperature greatly exceeding 212 may be attained, and 

 many substances ore thus dissolved which are otherwise 

 insoluble. 



Tho fact that water boils at a lower temperature if tho 

 pressure on it be diminished may easily bo proved experimen- 

 tally. Pour somo water into a flask, and place it over a spirit- 

 lamp till it boils ; when the steam is issuing freely, remove the 

 lamp, and cork tho flask tightly. After a few minutes pour 

 a stream of cold water on the outside, and ebullition will imme- 

 diately re-commence. The steam had expelled the air, the upper 

 part of the flask being filled with watery vapour of the same 

 tension. The cold, however, condensed this, and thus a partial 

 vacuum was produced, and the pressure diminished, in conse- 

 quence of which the water began again to boil. 



When water is converted into vapour much heat is rendered 

 latent. In this way the porous water-bottles, so frequently 



used in hot weather, act A portion of their content* slowly 

 percolate* the nnglaiad war* and evaporate* from the surface, 

 absorbing from tbo ve***l the beat required to convert . 

 vapour. 



If ether or any volatile liquor U dropped on the hand, a 

 sensation of cold will be at once produced, and this will be felt 

 more distinctly if the hand be waved About, or a current of air 

 driven over it, ao a* to accelerate the evaporation. The aaiM 

 thing occur* to a le*a extent with water. An important apj.U- 

 cation of thi* fact w now made in surgery. A stream of finely' 

 divided ether *pray i* blown npon any part of tb* body, and by 

 it* rapid evaporation produce* oold enough to freeze the flash, 

 and thus render it insensible to tho cut of the surgeon's knife. 

 In minor operation* this plan of producing local insensibility 

 is frequently adopted. 



By tho arrangement shown in Tig. 15 water may be frosts 

 by it* own evaporation. A shallow vessel, filled with strong 

 sulphuric acid, i* placed under tbe receiver of an air-pump, and 

 over it i* supported a thin metal vessel, A, containing water. Aa 

 soon as the air is exhausted, vapour begins to rise, and the 

 vessel would speedily beoome charged with it, did not the acid 

 absorb it a* fast a* formed. Each fresh portion of vapour 

 lowers tbe temperature, and this continued abstraction of beat 

 soon turn* tbe water into a lump of ice. 



Some vapour is given off at temperatures far below tbe boiling 

 point. Tho air, in fact, is always more or leas charged with it. 

 There is, however, a certain limit to the amount it can contain 

 at any temperature, and if, when it is fully charged, the tern* 

 perature fall, a portion of the vapour i* precipitated in tb* 

 form of rain. 



The point at which this vapour in tho air begin* to be pre- 

 cipitated is called the dew-point, and the temperature of thi* 

 depends npon tho amount of vapour present. When on a clear 

 night any object* become cooled below thi* point, tbe air in 

 contact with them deposits ita moisture, and they become wet 

 with dew. Hence, aa will bo soon, those object* which radiate 

 heat most freely receive most dew. 



Fig. 1C shows the instrument used for ascertaining tbe dew- 

 point. A gloss tube has a bulb blown at each end, and one of 

 them, A, is partly filled with ether. Thi* ho* been boiled and 

 the tube sealed while the vapour was issuing, so that no air is 

 present. Inside this limb is a delicate thermometer ; tho other 

 limb, B, is wrapped round with muslin. Ether is now dropped 

 upon this, and by its evaporation lowers the temperature. A por- 

 tion of tho ether in A therefore distils over, and its temperature 

 therefore diminishes likewise. As it sinks the bulb is watched, 

 and the thermometer read at the moment when vapour begin* 

 to form on it. To make this more clearly visible, the bulb i* 

 often mode of block gloss. An ordinary thermometer on tbe 

 stand of the instrument shows the temperature of tbe air, and 

 the difference between the two is thus easily noted. 



By means of pressure and cold several gases have been 

 liquefied. Carbonic acid, when exposed to a pressure of about 

 thirty atmospheres, becomes a liquid, and if this be allowed to 

 escape into the air, it freezes by ita own evaporation, and 

 becomes converted into flakes resembling snow. When these 

 are mixed with other, the evaporation is very rapid, and an 

 intense degree of cold is produced, so that mercury may easily 

 be frozen by means of it. 



Another effect of heat is to produce light. Ordinary flame 

 affords an illustration of this fact, the boat arising from the 

 chemical combination being the source of the light. Metals, 

 too, when exposed to a high temperature, beoome Inminon*. 

 A low red heat is usually assumed at from 1,100 to 1,300, 

 while a dazzling white indicate* from 2,500 to 3,000. Them 

 is, however, great difficulty in measuring these high tempera- 

 tures with any degree of accuracy. Wedgewood's pyrometer 

 is sometimes employed for tho purpose ; it consists of metal bars 

 placed about half an inch apart at one end, but a little nearer 

 at the other. Clay cylinders are then made of such a size that, 

 when baked at a temperature of 212, they just fit tbe wider 

 end. When exposed to a very high temperature, they contract, 

 and the extent of the contraction is shown by the distai 

 pass between the bars. The air thermometer is, however 

 reliable in its indications. A platinum vessel filled with air . - 

 exposed to the source of heat, and the expansion ascertained by 

 suitable means ; from this the temperature is easily deduced. 



If a powerful electric current be made to pass along a this 



