118 Progress in Science. (January, 
safe keeping of valuables, and even the dete¢tion of crime, in interference with 
property, when the progress made shall have rendered automatic photography 
practical, which is already possible. 
Mr. Joseph Sidebotham has given an account of a microscopical examina- 
tion he had made of dust blown into a railway carriage in which he was 
travelling near Birmingham. Having collected a quantity of the dust by 
spreading a newspaper on one of the seats near the open window of the 
carriage, Mr. Sidebotham brought his microscope to bear on it, and thus 
describes the result :—‘‘ With two-thirds power the dust showed a large pro- 
portion of fragments of iron, and on applying a soft iron needle I found that 
many of them were highly magnetic. They were mostly long, thin, and 
straight, the largest being about o15 of an inch, and under the power used, 
had the appearance of a quantity of old nails. I then with a magnet sepa- 
rated the iron from the other particles. The weight altogether of the dust 
collected was 57 grains, and the proportion of those particles composed wholly 
or in part of iron was 29 grains, or more than one-half. The iron thus sepa- 
rated consisted chiefly of fused particles of dross or burned iron, like ‘ clinkers ;’ 
they were all more or less covered with spikes and excrescences, some having 
long tails, like the old ‘Prince Rupert’s drops;’ there were also many small 
angular particles like cast-iron having crystalline structure. The other portion 
of the dust consisted largely of cinders, some very bright angular fragments of 
glass or quartz, a few bits of yellow metal, opaque white and spherical bodies, 
grains of sand, a few bits of coal, &c. I think it probable that the magnetic 
strips of iron are lamine from the rails and tyres of the wheels, and the other 
iron particles portions of fused metal, either from the coal or from the furnace 
bars.” 
Some important experiments have been published by Dr. Budde, from which 
anew theory of the photographic latent image may be deduced. Chlorine gas 
is passed into a tube closed at one end, and the gas is confined by a column of 
oil of vitriol saturated with chlorine. This must be done in comparative 
darkness. A beam of light is then decomposed by means of a prism, and the 
several coloured rays of the spectrum are allowed to fall in succession on the 
tube containing the chlorine, an arrangement having been made by which any 
alteration in volume that might take place in course of the experiment can 
be detected and carefully registered. When the red rays fell upon the tube, 
the effe& produced was very slight, the increase in the length of the gas 
column being only the #,th of an inch. According to the degree of refrangi- 
bility by the ray to which the chlorine was subjected, so did its expansion 
increase, until when under the action of the violet, the effect’ was at its 
maximum, the expansion being ten times greater than what was caused by the 
action of the red rays. What is ascertained from this experiment is, that the 
expansion of the gas is not due to heat, for were that the case the red rays 
would have exercised the most powerful action, this point having been further 
ascertained by delicate thermometers. To further establish the fa& that the 
expansion is due solely to the action of light, and not to a decomposition of 
the sulphuric acid by the chlorine, there was substituted for this acid, 
saturated with chlorine, the tetrachloride of carbon, the same result being 
obtained. The result of the experiment appears to warrant the conclusion 
that the violet rays of the sunbeam act by decomposing the molecule of the 
chlorine, setting free the two component atoms of which the molecule is 
supposed to be built up. The two atoms occupy a greater space when 
separate than when combined, and are also in a favourable condition for 
entering into combination. 
The estimation of the distance of the fixed stars has hitherto baffled the 
skill of the astronomer. Mr. Fox Talbot has proposed the following manner of 
effecting this obje@. Suppose the plane of the orbit of a binary system to 
pass through the sun, 7.e., that the observer is in the plane of the orbit, and 
that in the spectra of the individual stars there are lines belonging to the same 
element. The spectra of the two stars, taken through the same slit, should be 
observed and compared. When the stars appear in the same straight line, it 
