306 Analysis of Scientific Books, 



stitutes the total effect. It is this kind of heat which has been 

 employed as a principle of photometry, on the assumption that it 

 is precisely proportional to the intensity of light. Within certain 

 limits this may be the case ; but there are unquestionably circum- 

 stances under which the relation is very different ; sucli, for ex- 

 ample, as difference of colour in the light : and in general it can- 

 not be assumed to hold good in light from different sources. To 

 show this, there is a remarkable instance in incandescent metal, 

 which produces but very faintly illuminating rays, yet its '' trans- 

 missible heat" is very considerable. " I have repeatedly," says 

 Mr. Powell, "tried the experiment with a small ' photometer,' 

 having one bulb painted with Indian ink, and the other plain ; 

 the bulbs being in a vertical line ; this instrument whether em- 

 ployed with or witliout its case, or a glass screen, always gave 

 an effect of about 10° in 30" at eight inches distance from a ball 

 of iron heated to the brightest point in a common fire." 



In making these last experiments, the effect was always 

 greater when the instrument was used without its case, or a glass 

 screen. This was no doubt in part owing to the greater action 

 of the simple heat now admitted to the instrument on the coated, 

 than on the plain bulb ; but it was also in part occasioned by the 

 circumstance, that the stem going to the upper bulb passes in con- 

 tact with the lower, and being a solid mass compared with the 

 thin bulb, is slower in acquiring heat, and therefore cools it, thus 

 increasing the apparent effect on the other. 



II. Essai Geognostique sur le Gisement des Roches dans les 

 deux Hemispheres, par Alexandre de Humboldt. 1 vol. 

 8vo. F. G. Levrault, Paris, 1823. 



It is known that Baron Alexander Humboldt is a great traveller, 

 and it is believed that he is a great Astronomer, a great Bota- 

 nist, a great Political Economist, and, to sum up all, a great 

 Philosopher. It remained to be proved that he was also the great 

 Geologist that he had been believed ; and the proof is, the book 

 before us. 



The Title, implicating the whole sphere, (which is obviously 

 produced by the addition together of two hemispheres,) 

 would have led us to expect the history of all the rocks of our 

 sphere, or globe, had we not been accustomed to the grandilo- 

 quism of our author ; and Ave ought therefore to inform our 

 readers, that th« " gisements" in question are confined to a very 

 small portion of Europe, and a much smaller one of America. 

 The other two hemispheres, Asia and Africa, will probably ap- 

 pear in a future volume. 



