to the Earthy Constituents presented to their Absorbing Surfaces. 263 



many imperceptible cliannels by which eartliy 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 exclnding 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 

 iiad 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. Front's Paper on the 

 phosphate of lime existing in the young chick before the egg is hatched. 



