on Evaporation. 409 
those which cease to lengthen, only when they cannot be 
penetrated with more water ; as, by the use of these, there 
cannot be any fallacy in respect of the most important phse- 
nomena to be determined, namely, the more or less inten- 
sity, according to certain circumstances, in the cause which 
affects the hygroscope placed in a medium. 
3. The word moisture, like some others which also have 
been applied to certain phenomena the causes of which were 
not determined, is ambiguous in common language, imply- 
ing sometimes the idea of a cause, and at other times that of 
an effect, without a proper determination ; that defect might 
be corrected by means of different words for the cause and for 
the effect : but neologisms are often so troublesome, and 
sometimes so arbitrarily introduced into languages, that I 
shall only endeavour to determine the sense of the word mois- 
ture, according to cases, so as to avoid ambiguity. 
4. Moisture, taken in a general sense, may be considered sim- 
ply as invisible water, producing observable phaenomena. Thus, 
in hygroscopic bodies, the quantity of water which expands 
them, and increases their weight, is concealed within their 
pores ; and in the ambient medium, that water which affects 
hygroscopic bodies, being there under the form of steam, is as 
invisible as air itself. 
5. But in respect of hygrometry, where moisture is consi- 
dered as having correspondent degrees in the medium, and in 
hygroscopic substances, that word requires a more particular 
determination, on account of those two different situations of 
invisible water. Moisture may be either totally absent, or 
absolutely extreme, both in hygroscopic bodies, and in the 
ambient medium ; which circumstance, on both sides, affords 
