Composition^ and Analogies of Rocks. 213 



sition from limestone to any other rock. In the newer strata, it 

 is equally easy to understand how this must happen, from the 

 irregular succession of So small a number of materials ; ajid how 

 some uncertainty of composition must often take place at the point 

 of cliange between different deposits. The subject indeed scarce 

 seems worthy of a general discussion, and the particular tran- 

 sitions comprise a subject not within the limits of this paper. It 

 is to be proved that the imaginary value attached to these tran- 

 sitions has arisen from the practice, far too general, of deducing 

 conclusions respecting the order of nature, from that made by a 

 mineralogist in his cabinet. Undoubtedly, a rich cabinet may be 

 made to produce every transition which the most arduous theorist 

 could desire ; but he will have far mistaken the real objects of 

 his geological pursuits, who shall make his drawer the typQ of 

 nature. 



J. M'CULLOCH. 



Aet. V. Supplementary Remarks to a former Paper o?i 

 Light and Heat. ^By Baden Powell, M. A., F.R.S. 



(1.) In a paper on light and heat from terrestrial sources, in- 

 serted in the last Number of the Journal of Science, &c. (§. 8, and 

 26,) I alluded to experiments on the heating power of the light 

 from incandescent metal, but without particularly stating^ any 

 such results. As the fact is perhaps somewhat remarkable, it may 

 not be improper here to mention, that I have always found the 

 effect exhibited to the amount of a rise of 10" or more in thirty se- 

 conds, with Leslie's photometer, from a ball of iron two inches di- 

 ameter, heated to the brightest degree in a common fire. But for 

 the sake of those who may wish to repeat the experiment, it may 

 be necessary to remark, that several precautions must be taken. 

 The instrument, if of what is termed the portable kind, that is, 

 having its bulbs in the same vertical line, and the stem of the 



