WET WINDOWS. 213 



Q. What is meant by " the INSENSIBLE 



PERSPIRATION ?" 



A. From every part of the human 

 body an insensible and invisible perspi- 

 ration issues all night and day ; not only 

 in the hot weather of summer, but also 

 in the coldest day of winter. 



Q. If the perspiration be both INSENSIBLE 

 and INVISIBLE, how is it KNOWN that there is any 

 such perspiration ? 



A. If you put your naked arm into 

 a clean dry glass cylinder, the perspira- 

 tion of your arm will soon condense on 

 the glass, like mist. 



Q. Why are CARRIAGE WINDOWS very SOON 

 covered with thick MIST ? 



A. The warm vapour of the carriage 

 is condensed the moment it touches the 

 cold glass, and covers it over with a 

 thick mist. 



Q. WHY is the glass window COLD enough 

 to condense the vapour of the carriage ? 



A. Because the inside of the carriage 

 is much warmer than the outside, and 

 the glass window is made cold by con- 

 tact with the external air. 



