1878. The Beneficial Influence of Plants. | gor 
7 
Before passing to the consideration of moisture in the air as a 
means of lessening terrestrial radiation, which we have designated 
as the indirect effect of transpiration, it seems appropriate to give 
a place to a few of the laws governing radiation in general. 
Radiation has been defined by Tyndall as a vibratory move- 
ment which begins in the ultimate particles of matter, and is pro- 
pagated through waves of ether. 
Different bodies absorb heat and radiate the same in degrees- 
varying with their molecular constitution; that is to say, some 
bodies disturb the ether to a greater extent than others. It is an 
all important and universal law, that the power of a body to 
absorb heat and its power to radiate heat are reciprocal. Heat 
rays are emitted from both luminous and non-luminous bodies. 
The theory now almost universally accepted is, that heat and 
light are similarly transmitted; a ray which will give rise to the 
sense of heat by falling on the surface of the body will, if it fall 
on the retina, produce the sense of light, in general terms. 
Though the ray itself is the same in both instances, it is called, 
in the one case, a calorific ray, and in the other a luminous ray, 
on account of the effect produced. The only advantage in > 
retaining these terms is convenience in communication. Bodies 
which allow the rays of light to pass freely through them are 
said to be transparent, on the other hand, bodies which allow 
radiant heat to pass through them are said to be diathermic. It 
number of this journal. The author of the letter not being aware, at the time of 
writing, that the present paper was in preparation. 
J. M. ANpers, M.D. 
My Dear Dr:—I have read your “ Transpiration of Plants” with much satisfac- 
tion. The amount of water exhaled is so greatly in excess of what I supposed to be 
the usual pina that it leads me to believe that the common opinion of physicians 
and laymen, that plants are injurious in the sick room, is wholly erroneous. I sa 
the common opinion—I might say the universal opinion—for I have seldom, if ever, 
be well read in the literature of the EEE for, in your gare paragraph you say : 
“ The practical advantage of keeping plants in occupied rooms, in which the air is 
generally drier than that outside, ie also, from the results obtained, received further 
demonstration.” 
m tempted to ask you where you have ever seen the “ advantage ” of the prac- 
` tice spoken of? I speak now of plants in the sick room. Your paper has brought 
> me new thoughts, and carried me back over a practice of half a century to see 
ration of the advantage of the practice I can bring forward. Physicians ` 
have often spoken to me against the habit of some people who have growing plants 
in the sitting room occupied by the family, and especially in rooms occupied by con- 
