VOL. LXX.] PHILOSOPHICAL TKANSACTIONS. JOl) 



and its polished surface made quite clean, and then laid out for 2i hours. Be- 

 fore the expiration of this time it began to draw frozen matter from the air, 

 which settled all over the polished surface in long parallel lines, which gradually 

 multiplied, till at length it was mostly covered with a thin lihn resembling a 

 spider's web. 



The evaporation shown in the first part of this experiment was probably owing 

 to the speculum not having been sufficiently cooled when the film was first com- 

 municated to it from the lungs, and to its being further heated by that very ope- 

 ration. In the 2d part of the experiment the evaporation seems to have stopped 

 when the heat in the metal which favoured the process was exhausted; that is, 

 when the speculum had arrived at the temperature of the ambient air; for after 

 that, no heat could pass from the metal in order to contribute to the evaporation. 

 But from the last part of the experiment, the true disposition of the air at that 

 time, relative to bodies as cold or colder than itself, seems to be determined, 

 namely, that of giving out or depositing hoar-frost. 



Exper. II. On Sunday night, Jan. 23, several things were laid out at the 

 observatory, such as sheets of brown paper, pieces of boards, plates of metal, 

 glasses of several kinds, &c. which all began to contract hoar-frost seemingly 

 as soon as each body had time to cool down to the temperature of the air. The 

 sheets of brown paper, being so thin, acquired it soonest, and when beheld in 

 candle-light they became beautifully spangled over by innumerable reflections 

 from the small crystals of hoar-frost which had parted from the air. Evident 

 symptoms of tiie same tendency of the air to deposit, occurred on all the former 

 nights of observing, by which the tubes of the thermometers were so much 

 stained, that it required some attention to keep that part which corresponded to 

 the scale quite clear. 



These experiments indeed rather favour the opinion of the excess of cold, at 

 present treated of, depending on a principle the very reverse of evaporation. But 

 till opportunities offer in this, or in a colder climate, of making more experi- 

 ments, it will be too early to say any thing decisive concerning the nature or ex- 

 tent of a cooling process, which has so recently come under observation. All 

 that can at present be affirmed is, that in certain circumstances such a process 

 goes on, and that it depends probably on principles different from evaporation or 

 chemical solution. At the same time some of the experiments show that a free 

 communication between the hoar-frost and external air, perhaps while in motion, 

 is necessary; but in what manner this promotes the refrigeration does not as yet 

 appear. 



It would be going too far were we to conclude, from the experiments related 

 above, " that very cold air is never disposed to deposit its contents except on 

 bodies as cold or colder than itself" And yet that this is frequently the case 



