By persons of acute powers of smelling it may be recognised when there is one part of gas in five 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 substances, containing sulphur and hydrogen. These give off a gas called sulphuretted hydrogen, from which the fætid 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 be considered highly dangerous?
Not if it is intelligently managed. The appliances for the regulation 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.
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.