468 Mr. Wilson’s Account of a moji 
gradually multiplied, till at length it was moftly covered 
with a thin film refembling a fpider's web. 
The evaporation fhewn in the fir ft part of this experi- 
ment was probably owing to the fpeculum not having been 
fufficiently cooled when the film was firft communicated 
to it from the lungs, and to its being further heated by 
that very operation. In the fecond part of the experi- 
ment the evaporation fee ms to have flopped when the 
heat in the metal which favoured the procefs was ex- 
haufted ; that is, when the fpeculum had arrived at the 
temperature of the ambient air, for after that no heat 
could pafs from the metal in order to contribute to the 
evaporation. But from the laft part of the experiment, 
the true difpofition of the air at that time, relative to 
bodies as cold or colder than itfelf, feems to be deter- 
mined, namely, that of giving out or depofiting hoar- 
froft. 
EXPERIMENT XI. 
On Sunday, night, January 23, feveral things were 
laid out at the Obfervatory, luch as fheets of brown 
paper, pieces of boards, plates of metal, glafles of 
feveral kinds, &c. which all began to contract hoar- 
froft feemingly as foon as each body had time to cool 
1 down 
