1 98 
Relation of Moisture in Air 
[April, 
insensible perspiration, which will be greatly affeCted by the 
condition of humidity of the surrounding air. Here clothing 
becomes an important element. We protect ourselves 
against the inclemency of winter, or the heat of summer, 
by coverings more or less non-conducting or non-radiating, 
leaving but a small portion of the person unprotected by 
direbt exposure. An almost instinctive preference is given 
by all people, of all times, and at all places, for porous 
clothing; even the skins which clothe the inhabitants of the 
coldest regions, although quite impervious to moisture and 
to currents of air, are very open for the passage of vapour of 
water, or of diffused gases. Evaporation, and consequent 
cooling of the skin, takes place in great measure, or is in- 
fluenced by the relative vacuum which the quantity of 
vapour present in the air establishes. The transfer of vapour 
is then one of diffusion, and follows the law of diffusion of 
gaseous bodies. A partially anhydrous air, external to the 
clothing, is a partial vacuum to the vapour of go°, existing 
in duCts of perspiration, and this vapour rushes towards the 
vacuity without encountering the resistance of any circula- 
tion, and meet no considerable obstacle in the porous coats 
and overcoats. It is in this way possible to explain why, 
in mid-winter, with the room from 65° to 7 o°, heavy under- 
clothing is not only endurable, but necessary. The overcoat 
may be removed on entering the well-warmed house, but no 
discomfort follows from the retention of warm garments 
that, with a summer condition of air of the same tempera- 
ture would be oppressive. We sleep in rooms which, if 
not warmed to the full heat of our living rooms, have yet 
the “ temperate ” point of indication by the thermometer, 
and in this case enjoy a pile of bed-clothing, which would 
be suffocating in weather of the same natural temperature. 
The A.merican requirement of comfortable drawing-room 
clothing is strikingly different from that of England. The 
ladies’ English drawing-room dress is an impossibility in 
America. Even the rigorous laws of fashion fail to conform 
themselves in this case, and yet our American drawing- 
rooms are hotter than the English ones. 
What proportion of the heat generated by formation of 
carbonic acid to be dispersed from the body after taking out 
what is abstracted by exhalation and by labour of work or 
animal life is expended in vaporisation of water is of 
course doubtful. Some authorities give 2 to 2J lbs. of 
water to be evaporated each day by insensible perspiration. 
These quantities would give (nearly) 2000 to 2500 units, 
or i-J to if units of heat per minute, and, together with the 
