SOME EXPERIMENTS ON THE SAP OF PLANTS. 259 
ficent nature gathers together that warmth, and smoke and ash, 
and returns them to us in the form of trees and flowers, at the 
same time presenting us with the exact amount of life-giving air 
we had lost. How do plants grow? By additions of carbon, 
nitrogen, the elements of water, and certain mineral elements. 
Whence comes their food? From its storehouses, the atmosphere 
and the earth. How is it conveyed? By the carriers, air and water. 
But these answers to the questions respecting the growth of 
plants, though very direct, are very crude and general. They 
represent only the beginning and the end of a vast number of pro- 
cesses. A log of wood on the one hand and the products of its 
burning on the other are only the terminals of two long series of 
substances, an analytical series beginning with the wood and ending 
with gases and ashes, and a synthetical series beginning with gases. 
and ashes and ending with the wood. Reduce the wood to its 
elements, not by the rough and rapid method of free combustion, 
but by controlled heat in retorts, as at gas-works, and you will 
obtain a series of substances which includes scores of interesting 
materials—acids, spirits, colour-yielding bodies, illuminating agents, 
etc. Conversely, nature, between the constituents of the air or of 
the soil on the one hand, and the finished tree or flower on the 
other, forms a series of substances which includes scores of inter- 
esting materials—acids, perfumes, colours, flavours, starch, sugar, 
oil, etc. 
In these two series or chains the products are related to each 
other or linked together, but in what way we do not yet perfectly 
know. Indeed, as to the form or character of many of the links 
themselves we as yet know nothing. But chemists discover fresh 
links every month, and chemistry is revealing to her students some 
hints as to the manner in which they are joined. So that man’s 
information on these matters is increasing year by year, and what 
is revealed gives great encouragement to further research. 
More fascinating, possibly, than the study of the chemistry of 
plant-growth, either from the analytical or from the synthetical 
points of view, is the study of the natural philosophy, as it used to 
be termed, or physics or true science of the subject ; that is to say, 
the study of the cause of the various effects, or a consideration of 
the explanation of the effects. Bring a log of wood and air together 
under proper conditions and wood is reproduced, but one of the in- 
dispensable conditions here is that heat must be absorbed, the sun 
must shine on the leaves of the tree producing the wood. What 
happens to the heat that is absorbed before it again shows itself 
when the log is, in the grate, once more resolved into its elements ? 
Does it lie dormant in the wood, latent, and for the time useless ? 
Matter is always in motion; surely force is never quiescent. 
Neither matter nor force can suffer destruction, but each is ever 
