220 Miscellaneous Intelligence. 



moves continually in a manner more or less regular, without, how 

 ever, any sensible change in the place of any one point. 



Whatever the cause may be, it appears from the experiment that 

 the continued action of light on a single point of the retina, occasions, 

 in certain circumstances, a sensation similar to that which would 

 r£sult from the same light acting successively on several points in 

 the immediate vicinity of each other. — B. Prcvost, Annales de Chi- 

 mie, xxxvi. 432. 



II. Chemical Science, 



1. Heat evolved during Combustion. — M. Despretz has lately 

 read some memoirs on the heat evolved during combustion. He 

 found that hydrogen is the body of which a given weight gives out 

 most heat, and the metals the least. But the result is of the oppo- 

 site kind if referred to equal weights of oxygen. Carbon, which, in 

 burning, does not alter the volume of the oxygen gas it consumes, 

 produces three-fifths of the heat evolved by the metals, iron, zinc, 

 and tin, which reduce the oxygen to the solid state. Hence it is in 

 the act of combination that we must seek for the principal cause of 

 the developement of heat, and not in the approach of the particles. 

 M. Despretz has also found that the quantity of heat developed by a 

 certain quantity of a body which burns without changing the vo- 

 lume of the gas is the same whatever be the density of the gas. — 

 Le Globe, Phil, Mag. N. S. 



2. Conducting Tower for Heat of the Pri?icipal Metals, and some 

 Earthy Substances. — The following results have been obtained by 

 M. Despretz, the experiments having been made with extreme care. 



Gold 1000.0 



Silver 973.0 



Platina 981.0 



Copper 898.2 



Iron 374.3 



Zinc 363.0 



Tin 303.9 



Lead ...c 179.6 



Marble 23.6 



Porcelain 12.2 



Fire-brick 1 1.4 



That platina should be above copper, and even silver, induces us 

 to insert the description of the experiment. All the bars used were 

 square prisms. Cavities were made in them at equal distances of 

 10 centimetres, to receive the bulbs of small thermometers. The side 

 of the section, except for the two last bodies in the list, was equal 

 to 21 millimetres. The bars were covered with the same varnish, 

 to give them an equal radiating power. The bar experimented 

 with, was heated at one extremity by a small stove, which has the 

 advantage of being governed readily, and of causing but little heat 



