4-8 



Case 1st.— On the Evaporation of Fluids. 



Dr. Cullen has sufficiently proved, in his papers in the Edinburgh Essays, 

 that cold is produced by the evaporation of all fluids. There is also a circum- 

 stance daily before our eyes, which proves the same thing, though I do not 

 know that it has hitherto been taken notice of. 



It is well known that water, as soon as it begins to boil, continues exactly 

 at the same heat till the whole is boiled away, which takes up a very consider- 

 able time ; I believe I may say several times as much time as it took up to 

 heat it to the boiling point. No reason however can be assigned why the fire 

 should not continually communicate as much or nearly as much heat to it after 

 it begins to boil as it did when it wanted not many degrees of boiling, & yet 

 during all this time it does not grow at all hotter ; this I think shews that there 

 is as much heat lost, or in other words, as much cold produced, by the action 

 of boiling as there is heat communicated to it by the fire. 



If no cold was produced by the action of boiling, the water should either 

 grow hotter & hotter the longer it boils, or else it should be entirely converted 

 into steam immediately after it begins to boil. 



By this means when the water is heated to the boiling point, then as fast as 

 it receives heat from the fire there is immediately so much of the water turned 

 into steam as is sufficient to produce as much cold as it receives from the fire, 

 so that the water is prevented from growing hotter, & besides the water will 

 not be entirely evaporated till it has received as much heat from the fire as is 

 produced by turning the whole of the water into steam. The foregoing cir- 

 cumstance, I think, shews that all animal & vegetable substances, sulphur, 

 quicksilver, arsenic, & many other metallic & other substances generate 

 cold by being changed into an elastic fluid or vapour; though it cannot be 

 shewn by Dr. CuUen's method that they do so, for in distilling any of these 

 substances it takes up a vast deal of time, after they begin to distil strongly, 

 before they are all driven over, & there is no reason to think that their heat 

 increases much after they begin to distil strongly. In general I think there is 

 great reason to suppose that all the substances whatever, which are capable of 

 being volatilized by heat, produce cold thereby. 



Case 2nd. — I have been informed, that in distilhng water and other liquors, 

 the water in the worm-tub is heated thereby much more than it would be by 

 mixing with it a quantity of boiling water equal to that which passes through 

 the worm. 



PAPER ON HEAT. 



p. 1. It seems reasonable to suppose that on mixing hot & cold water, the 

 quantity of heat in the liquors taken together should be the same after the 

 mixing as before, & that the hot water should communicate as much heat to 

 the cold water as it lost itself, so that if the expansion of the mercury in the 

 thermometer is proportional to the increase of heat, the difference of the heat 

 of the mixture, & of the cold water, as measured by the thermometer, multi- 

 plied by the weight of the cold water, should be equal to the difference of heats 

 of the hot water & mixture, multiplied by the weight of the hot water ; or 



