Improvements in Science. 119 



length of the undulation of the extreme red and extreme 

 violet. 



The phenomena of absorption which take place in bro- 

 mine gas and in potash-oxalate of chromium, observed by 

 Brewster, are explained in the same manner; and, the 

 differences which exist between these phenomena, only 

 proceed from differences between the absolute values of 

 the retardations. In order to explain the phenomena 

 noticed in nitrous acid and euchlorine, it is necessary to 

 admit more than two retardations. This difference is not 

 remarkable, because the gases are compounds. The spectra 

 derived from certain coloured flames, are explained in the 

 same manner as the phenomena of absorption. But, besides 

 the supposition of retardations, it is necessary to admit, 

 that certain flames give only certain kinds of light. Some 

 of the phenomena of this kind of spectra proceed only from 

 such a cause ; such is the case, for example, in the orange 

 tint observed in the spectrum, proceeding from the flame 

 of a candle. Wrede has proved, that this tint, proceeds 

 from the circumstance, that the exterior part of the flame, 

 where true combustion takes place, affords an absolutely 

 homogeneous light, and, consequently, undergoes no dis- 

 persion. 



III. ELECTRICITY AND MAGNETISM. 



1. Method of determining the electrical conductibility of 

 small masses. — -The usual method of determining this pro- 

 perty in bodies, consists in interposing between an electrical 

 source and a metallic wire attached to a sensible electro- 

 scope, the body whose conductibility is to be ascertained. 

 For this purpose, an electrical machine, a voltaic or a 

 dry pile is employed. Several ingenious apparatus have 

 also been substituted. Lassaigne recommends a modifica- 

 tion which he has found to answer. To one of the wires of 

 Schweigger's multiplier, he attaches a small platinum spoon, 

 containing dilute nitric acid ; above this spoon, is fixed 

 upon a support, a small glass tube, 2-3 inches long, and 

 •19 inch in diameter. A wire of red copper curved at one 

 of its extremities, traverses it for two-thirds of its length. 

 To this distance the wire is flattened into a spatula, or ter- 

 minated by a disk. To this part of the wire the body to 



