to the Earthy Constituents presented to their Absorbing Surfaces. 263 
many imperceptible channels by which earthy and alkaline matter may obtain 
admission to the juices of a plant*. 
Had I not very early in the course of these experiments been led to despair 
of excluding the minute but continual supplies, which are probably brought 
by the very air and water which come into contact with the absorbing sur- 
faces of every vegetable, especially in the centre of a large town, I should not 
have remained satisfied without purifying the sulphate of strontian in which 
the seeds were sown from the other earths with which I found it to be mixed. 
But the labour of getting rid of these ingredients seemed to be uncalled for 
with reference to the objects to which I found it necessary to confine my in- 
quiries ; since even had I employed the earth in a state of perfect purity, and 
detected an excess of lime in the plants reared in it beyond that contained 
in their seeds, still I should not have been justified in inferring the actual 
generation of earthy matter, any more than I have felt myself to be from the 
similar result I obtained when flowers of sulphur were the matrix in which 
the plants had vegetated. 
The faculty, however, possessed by them of rejecting strontian, even when 
presented to the absorbing surfaces of their roots in a state of solution, would 
seem sufficiently substantiated; and an analogous circumstance may be cited 
in the animal kingdom, if I can rely upon an experiment which I made several 
years ago, that of confining some hens of the Guinea-fowl during the breed- 
ing-season in a place where they could obtain no other earth, except some 
powdered sulphate of strontian, which they appeared to devour greedily. 
Yet only a minute trace of this earth was discoverable in the shells of their . 
eggs, of which those laid during the first part of their confinement retained 
their natural hardness, but those of later production were as soft as if the birds 
had been entirely debarred from every kind of earthy matter. 
It may be asked, whether the strontian is taken first into the system, and 
afterwards excreted from it, or whether the spongioles of the roots refuse it 
* 'The case which I should be most disposed to bring forwards in support of the contrary opinion 
is that of the phosphoric acid, which forms so abundant an ingredient in all animal structures. Is its 
quantity sufficiently accounted for by that introduced into the system by the food taken in? On this 
subject I hope at some future time to complete some experiments. See also Dr. Prout's Paper on the 
phosphate of lime existing in the young chick before the egg is hatched. 
