i 
804 The Beneficial Influence of Plants. [December, 
substance in the foremost ranks as an interceptor of radiant 
caloric. 
These researches by Prof. Tyndall were extended, also, to 
vapors of different bodies, such as sulphuric, boracic and formic 
ethers; and determined them to be highly active in interfering 
with calorific rays, boracic ether “ exceeding any other substance 
hitherto tried.” All the experiments above quoted are certainly 
of great scientific interest and importance ; but happily this illus- 
trious scientist did not omit to study the effect on radiation of the 
aqueous vapor constantly in our atmosphere. The quantity of 
vapor of water contained in the air is, however, very small 
indeed, constituting only about four anda half per cent., and, 
although the moisture is everywhere present, its ratio is very 
variable. It is perfectly invisible, so that by our senses we are 
quite unable to judge of the amount present ; even the purest sky 
may contain a large proportion. As this vapor is to all intents 
and purposes a gaseous body, obeying the laws of gases, any one 
not familiar with the information which we have just outlined, 
would, doubtless, hesitate to accept the assertion that the watery 
vapor so sparsely scattered through the atmosphere is the main 
agent in regulating the nocturnal radiation from the earth’s sur- 
face. Even Tyndall, himself, neglected for some time this sub- 
stance; and, in his own phraseology, “could hardly credit the 
first result, which made the action of the aqueous vapor of the 
laboratory fifteen times that of the air in which it was diffused.” 
But this result does not show the correct relation of the action of 
vapor and air; for after repeated experiments with air from 
different localities and examined similarly, the results were 
uniformly to the effect that vapor of water has an absorbing 
capacity seventy times that of the air in which it is contained. 
Many objections and criticisms, some of which seemed almost 
insurmountable, were overcome by varying the methods of pro- 
cedure. The assertion made above, therefore, seems to have been 
fully and satisfactorily demonstrated by a most careful and com- 
petent experimentalist. 
Still further testimony might be adduced, the result of obser- 
vations of meteorologists. Col. Richard Strachey, an eminent 
meteorologist, made observations showing the relation between the 
~ tension of the aqueous vapor of the atmosphere and the fall of 
_ the thermometer during the night. A single statement taken 
