HEAT. 



meter of one-eighth of an inch; the 

 tube is bent into right angles near the 

 ends, both of which are terminated 

 by bulbs (Jig. 20.) One of the bulbs 



Fig. 20. 



is nearly half filled with water, which 

 being made to boil, the air is expelled 

 from the tube and bulbs, which remain 

 filled with steam ; the open bulb is 

 then closed by melting the glass at its 

 capillary termination. The instrument 

 having been thus prepared, is ready for 

 Use. When the empty bulb is im- 

 mersed in a mixture of salt and snow, 

 the vapour existing within it is con- 

 densed, a vacuum is produced, which, 

 removing pressure from the surface 

 of the water in the other bulb, 

 enables it to evaporate rapidly; the 

 vapour being condensed as fast as it is 

 produced, the water is therefore speedily 

 frozen, although it may be at the dis- 

 tance of two or three feet from the 

 bulb which is immersed in the cooling 

 mixture. 



When the human frame has been 

 exposed to a considerable degree of 

 heat, and the clothes have become 

 moist with perspiration, danger is in- 

 curred by sudden and continued ex- 

 posure to air of a lower temperature, 

 on account of the cold produced by 

 evaporation, by which the body is liable 

 to be too much reduced in temperature. 



The bulb of a mercurial thermome- 

 ter, being surrounded with cotton or 

 tow, kept moist with ether, and ex- 

 posed to a current of air, the tempera- 

 ture of the quicksilver may be reduced 

 far below zero of Fahrenheit's scale.. 



A small animal would be deprived 

 of its vital heat, in a very short time, 

 by exposure to a current of air, while 

 wet with ether. 



Ex. The instrument called a pulse- 

 glass, (fig. 21,) is a glass tube with 



Fig. 21. 



a bulb at each end of the form re- 

 presented. It is partly filled with 

 coloured spirit of wine, and partly with 

 air ; when it was closed by the blow- 

 pipe, a portion of the^ spirit of wine had 



been converted by heat into vapour, 

 which occasioned a part of the air to 

 be expelled, so that the air remaining 

 within is very thin or rare. When the 

 bulb which contains the liquid is 

 grasped in the hollow of a warm hand, 

 the air above the liquid and the vapour 

 mixed with it are expanded, and press- 

 ing upon the liquid, force it over to 

 the other side ; the bulk of the liquid 

 having passed over, a rapid bubbling, 

 similar to boiling, instantly commences ; 

 a portion of spirit of wine remains ad- 

 hering to the surface of the bulb ; this 

 small portion, being converted into 

 vapour, passes over along with a por- 

 tion of the expanded air, and rising up 

 through the liquid, occasions the ap- 

 pearance alluded to. There is yet 

 another circumstance attending this 

 experiment which requires to be men- 

 tioned and explained. As soon as the 

 bubbling begins, an intense sensation 

 of cold is felt in the hollow of the 

 warm hand, on account of the heat 

 which is suddenly withdrawn, to enable 

 the spirit of wine to assume the state 

 of vapour; for this change cannot be 

 effected until a quantity of sensible 

 heat enters into it, and becomes latent 

 or concealed heat. 



The air-thermometer (Jig. 5, page 15) 

 will answer well to illustrate the effect 

 of evaporation in producing cold. 



Ex, A little ether being poured upon 

 the bulb of this instrument, heat will 

 be absorbed during the evaporation, 

 to enable the ether to assume the ex- 

 panded state of vapour ; a part of this 

 heat being taken from the air within 

 the instrument, it contracts in bulk, and 

 permits the liquid to rise considerably 

 higher in the tube, indicating the 

 quantity of heat withdrawn. 



Distillation. 



In the process of distillation for the 

 purpose of obtaining alcohol, that fluid 

 having been produced by fermentation, 

 it is to be separated from the water 

 with which it is mixed ; and that distiller 

 is the most skilful who can separate the 

 largest quantity of alcohol at the least 

 expense, and without imparting any 

 disagreeable flavour. The principle 

 upon which the separation is effected 

 is the greater volatility of the alcohol, 

 or spirit of wine, as it is usually called ; 

 consequently it assumes the state of 

 vapour more readily than the water 

 with which it is mingled, This vapour 



