44?5 Mr. R. Acklams on the Action of 



phyll. The carbonate of lime is secreted through these 

 cavities in greater quantities the richer the soil is in lime. 

 We also, however, find the leaves of the above-mentioned 

 species of Saxifragcc very thickly covered with lime, when they 

 have become old, in a soil very rich in humus. It may also 

 frequently be observed, that many other places on the upper 

 surface of the leaves of those plants, besides the cavities, are 

 covered with a thin crust of chalk ; that the deposition therefore 

 of this calcareous substance does not take place through the ca- 

 vities only. Unger, it is true, says, " We should in this case be 

 mistaken if we were to consider the calcareous excretion as a 

 product of the entire epidermis. I myself, however, think that 

 this is the case; and it is easily observable on many garden 

 plants of this tribe. These excretory organs on the upper 

 side of the leaves, are said to be represented by an immense 

 number of pores on the under surface, as if the increased 

 process of secretion on the one side had produced an antago- 

 nistic process on the other side, but different in quality." In 

 fact, this deposit of lime upon the leaves of Saxifrages is an en- 

 tirely peculiar phenomenon, and can be connected with a few 

 others only; it cannot even be viewed as parallel with the incrus- 

 tation of the Chares, for in these the lime appears to be precipi- 

 tated from the surrounding water, as the carbonic acid, which 

 held it in solution, is imbibed by the plant. In the Saxifrages 

 only an exuding of the calcareous fluid seems to take place, and 

 this is strongest in those cavities where the cellular tissue is 

 very delicate : the phenomenon may be classed with the de- 

 position of lime in the air-cavities of the leaves of Lathrcea> 

 and with the occurrence of crystalline druses on the sides of 

 the air-ducts in Myriophyllum. We also often find in other 

 plants a secretion of a salt which is contained in the soil on the 

 surface of the leaves, &c* 



[To be continued.] 



LVII1. On the Action of Cold Ah' in maintaining Heat. By 

 Robert Addams, Esq., Lecturer on Natural Philosophy, fyc. 



To Richard Phillips, Esq. 

 Dear Sir, 



IN the last Number of the Philosophical Magazine, p. 407, 

 there is an account of the "Action of Cold Air in main- 

 taining Heat." I conclude, from the initials R. P. being an- 

 nexed, that you are the author of that communication ; I there- 

 fore suppose you will be the more immediately interested in 



* Who first called attention to the occurrence of lime, on the leaves of 

 Saxifrages ? 



