16 FIRST PRINCIPLES. 



when it cannot be seen on the skin by the unaided eyes ; and 

 sweat or sensible perspiration, when it is thus visible. The 

 term, perspiration, is applied generally to both forms. The 

 secretion of perspiration is regulated by the quantity of blood 

 supplied to the skin, and also by direct stimulation of the 

 nerves of the glands that secrete this fluid ; both forms of 

 control being exercised by the nervous system. Perspiration 

 may be excited by muscular exercise, external heat, various 

 poisons (including those of certain diseases), emotion, and 

 other kinds of nervous disturbance, which may be indicative 

 of bodily weakness or ill health. The drinking of water 

 tends to promote perspiration by increasing the volume of 

 the blood. On the other hand, as drinking water is generally 

 much colder than the body, the ingestion of the former will 

 as a rule lower the temperature of the latter. Hence, the 

 warmer the water which is drunk, the more will the secre- 

 tion of perspiration be excited ; and vice versa. Speaking 

 generally, we may say that the appearance of sweat is 

 compatible with health only when it has been brought on 

 by an increased supply of blood. When sweat breaks out 

 independently of the circulation, the skin, as a rule, being free 

 from an excess of blood and being cooled down by the 

 process of evaporation that is in action on its surface, will 

 be unusually cold ; hence we may regard the existence of a 

 " cold sweat " as a sign that the health of the animal is more 

 or less upset. The hot and dry condition of the skin in cer- 

 tain stages of intermittent fever, for instance, is an example 

 of the arrest of perspiration by a poison (toxin) produced by 

 the presence of disease germs in the system. 



We have seen (p. 12) that evaporation has a cooling effect 



on the surface from which it takes place ; because a liquid 



to be converted into vapour, has to acquire a certain quantity 



of heat, which it naturally absorbs from its surroundings. 



Evaporation is dependent on the capability of the air to 



