THE SKIN. 199 



a cold mirror, which will condense the invisible vapor and 

 soon be covered with a slight dew ; or if we put the hand 

 into a large tumbler or glass pitcher previously wiped dry, 

 and wind a towel about the wrist, so that nothing can pass 

 out at the mouth of the glass, we shall then soon see the 

 moisture gather upon the inner surface. This can be noth- 

 ing more than, the condensed exhalation from the hand. 



471. This is called the insensible perspiration, in distinc- 

 tion from the sensible or visible perspiration, which flows in 

 drops from the skin when we are excited or are unusually 

 warm. The insensible perspiration never ceases to flow dur- 

 ing health ; and under all circumstances, if the skin is in a 

 good condition, it is not interrupted. But the sensible per- 

 spiration flows only occasionally, and, though more abundant 

 sometimes than the other, yet the whole amount is much less. 

 In the cold-blooded animals, the toad, serpent, &,c., the 

 insensible is six times as great as the sensible perspiration. 

 The difference in man is not so great as this, yet it is con- 

 siderable. 



472. The sensible perspiration the sweat is at times 

 very great, and occasionally we saturate our clothing with it 

 in a very short period. Some experiments were performed, 

 and observations made, at the Phcenix Gas Works, in London, 

 Nov. 18, 1836, to determine how large a quantity would be 

 thus thrown out from the body under favoring circumstances, 



473. " Eight of the workmen regularly employed at this 

 establishment in drawing and charging retorts and in making 

 up the fires, which labor they perform twice a day, commonly 

 for the space of one hour, were accurately weighed in their 

 clothes immediately before they began and after they had 

 finished their work. On this occasion, they continued at 

 their work exactly three quarters of an hour. In the inter- 

 val between the first and second weighing, the men were 

 allowed to partake of no solid or liquid, nor to part with either. 

 The day was bright and clear, with much wind. The men 



