THE EEASON WHY. 39 



" From the place of his habitation he looketh upon all the inhabitants of the 

 earth." Ps AIM xxxm. 



68. What proportion of gas in the air may be recognised 

 ly the smell ? 



By persons of acute powers of smelling it may be recognised when 

 there is one part of gas in Jive hundred parts of atmospheric air ; 

 but it becomes very perceptible when it forms one part in a hundred 

 and fifty. Warning is, therefore, given to us long before the point 

 of danger arrives. 



69. What other sources of hydrogen are there in our 

 dwellings ? 



It arises from the decomposition of animal and vegetable sub- 

 stances, containing sulphur and hydrogen. These give off a gas 

 called sulphuretted hydrogen, from which the fsetid effluviam of 

 drains and water-closets chiefly arise. We should, therefore, take 

 every precaution to secure effective drainage, and to keep drain-traps 

 in proper order. 



70. May the use of gas for purposes of illumination le con- 

 sidered highly dangerous ? 



Not if it is intelligently managed. The appliances for the regu- 

 lation of gas are so very simple and perfect, that accidents seldom 

 arise except from neglect. In England 6,000,000 tons of coal are 

 usually consumed in the manufacture of gas, producing 60,000,000,000 

 cubic feet of gas. And yet accidents are of very uncommon 

 occurrence. 



CHAPTER V- 



71. What is heat? 



Heat is a principle in nature which, like light and electricity, is 

 best understood by its effects. We popularly call that heat, which 

 raises the temperature of bodies submitted to its influence. 



72. What is caloric ? 



Caloric is another term for heat. It is advisable, however, to use 

 the term caloric when speaking of the cause of heat, and of heat 

 as the effect of the presence of caloric. 



