xiii 
of the assistance of Mr. Mallet, a gentleman peculiarly qualified for 
such investigations, both from his knowledge as a chemist, and from 
his opportunities of observation as a practical engineer. An extensive 
series of experiments has accordingly been instituted by him, with the 
support of the Association, on the action of sea and river water, in dif- 
ferent circumstances as to purity and temperature, upon a large number 
of specimens of both cast and wrought iron of different kinds. These 
experiments are still in progress, and the effects are observed from 
time totime. They will afford valuable data for the engineer, and form 
the principal object of the inquiry; but a period of a few years will 
be required for its completion. In the mean time, Mr. Mallet has fur- 
nished a report on the present state of our knowledge of the subject, 
drawn from various published sources, and from his own extensive ob- 
servations. In this report he examines very fully the general condi- 
tions of the oxidation of iron, and how this operation is greatly pro- 
moted, although modified in its results, by sea water; also in what 
manner the tendency to corrosion is affected by the composition, the 
grain, porosity, and other mechanical properties of the different com- 
mercial varieties of iron. The influence of minute quantities of other 
metals, in imparting durability to iron, is also considered. Mr. Mal- 
let devotes much attention to the consequences of the galvanic asso- 
ciation of different metals with iron, a subject of recent interest from 
the applications of zinc and other metals to protect iron, which are 
at present agitated. He concludes this, his first report, by recommend- 
ing a series of inquiries, ten in number, which will supply the desiderata 
immediately required by the engineer and by the chemist. 
We have next to notice a report by Professor Powell, on the present 
state of our knowledge of refractive indices for the standard rays of the 
solar spectrum in different media. The difficulty which the fact of 
the dispersion of light has offered to the universal application of the 
undulatory theory, has been in a great measure removed by the ana- 
lysis of Cauchy and others, who have considered the distances of the 
undulatory particles as quantities comparable to the length of a wave ; 
velocities of propagation of the different rays of the spectrum are 
made to depend upon the length of wave which constitutes a ray of a 
given colour, and upon certain constants proper to the medium; these 
constants being obtained from observations on refractive indices for 
certain definite rays (or dark lines) of the spectrum, the refrangibility 
of any other definite ray (whose wave-length has been ascertained by 
examining an interference-spectrum) becomes known, and may be 
compared with observation as a test of theory ; such experiments have 
been made by Frauenhofer, Rudberg, and Professor Powell, who has 
