SCIENTIFIC SUMMARY. 
125 
made use of a new method for indicating the point when a true chemical 
compound is obtained, lie examines various mixtures of two metals, com- 
paring their densities with that which would be expected were no com- 
bination to take place. In most cases it is found that, there is a contraction ; 
and the point of maximum contraction he considers to indicate the exist- 
ence of a true alloy. By this method he determined the composition of 
the alloy of tin and lead to be the same as was obtained by Rudberg in 
a totally different manner. 
Mining . — The lamentable accident at Hartley Colliery, in January last, 
gave rise to a general belief that two shafts are required to each mine for 
the purposes of ingress or egress. As a result of the outcry, an Act has been 
passed which compels colliery proprietors to have two separate means of 
exit from each mine, although the two openings need not belong to the 
same mine. Fortunately many other dangers are lessened by the use of 
two shafts, so that the compulsion cannot be considered a hardship. 
Many accidents have happened in coal-mine shafts from the breaking 
of the rope. A great variety of “safety-cages” have been invented, so 
arranged that if the rope breaks, a set of springs push out arms or levers 
which grip or wedge fast against the conducting-rods, so that the cage is 
brought to a stand. One difficulty, however, has arisen from the employ- 
ment — now becoming very general — of iron conducting-rods, whilst most 
“safety-cages” were suited for wooden ones. Two or three modifications 
have, however, lately been brought out, which are intended especially to 
apply to iron rods, and we hope that they will receive all attention from 
the colliery proprietors. 
ERHAPS some of our readers may think that a record of the various 
improvements which are made from time to time in the photographic 
processes would derive additional interest from a description of the results 
obtained by new applications to the art. Such an account would, how- 
ever, fill our pages, and leave room for no other subject; for in the present 
day, everybody and everything is photographed. Objects which require 
the highest powers of the telescope or microscope for their definition are 
brought before our eyes in a clear and agreeable picture. Others, again, 
are reduced from their natural size, and the photographic image is so 
diminished that the microscope must be employed to render it once more 
visible to the eye. Some of these works of art are simply curious and 
interesting ; others are also useful. Then, as to persons, we will not 
ask who is, but who is not, photographed nowadays? Indeed, we 
think, it will be obvious to all, that it would be impossible for us to give 
an account of the various results which are from time to time attained by 
means of photography. Nevertheless, when extraordinary instances of 
success are brought under our notice, such as we deem deserving of the 
attention of our readers, we shall at all times be glad to accord them 
a portion of our space. 
PHOTOGRAPHY. 
A TRIUMPH OF PHOTOGRAPHIC ART, 
