1868.] Physics. ‘ 263 
Society a Hand Spectrum-Telescope devised by him in the summer 
of 1866, for the purpose of observing the spectra of meteors and 
their trains. The apparatus consists essentially of a direct-vision 
prism placed in front of a small achromatic telescope. 
A paper “On the Action of Sunlight on Glass” has been pub- 
lished by Mr. Thomas Gaffield in ‘Silliman’s American Journal.’ 
Mr. Gaffield found the greatest change in the colour of glass to 
take place in the summer, the least in winter, and that in spring 
and autumn about equal, and midway between these extremes. 
Crystal or lead glass and a piece of optical glass, containing 
probably little, if any, manganese, suffered no change by two years’ 
exposure. Coloured glasses after two or three years’ exposure 
showed no perceptible change in any instance, excepting a slight 
one in a single purple specimen. Experiments made with artificial 
heat of various degrees of intensity showed the colour of glass to 
be unacted upon by heat; the same or similar specimens, almost 
without exception, undergoing change by a few months’ exposure 
to sunlight. Specimens exposed in hot water for a month indoors 
and out of sunlight experienced no change in tint; similar ones 
exposed during the same length of time in a dish with two or three 
inches of water out of doors, suffered a decided change, though 
only about half as much as when exposed directly, without the 
aqueous medium. Mr. Gaffield arrives at the conclusion that air 
moisture and artificial heat effect no change in the colour; the 
change appears to be due to the actinic rays of the sun alone. 
Heat.—Dr. J. P. Joule, F.R.S8., has described a thermometer 
unaffected by radiation. It consists of a copper tube about one foot 
long, having another tube open at both ends in the centre, and the 
annular space filled with water. In the inner tube there is a spiral 
of fine wire, suspended by a filament of silk, and having a mirror 
attached to it. ‘The lower end of the tube is closed by a lid, capable 
of removal at pleasure, and when this lid is removed, if the air in 
the tube have a different temperature from that of the outside 
atmosphere, a current of air and a consequent turning of the spiral 
will be the result. In Dr. Joule’s apparatus, one degree Fahrenheit 
produces an entire twist of the filament. He finds the temperature 
in the tube to be generally warmer than in the outside atmosphere 
of a room, owing to the conversion of light and other radiations into 
heat on coming in contact with the copper tube. This result is also 
manifested in the open air on a still day; when there is wind the 
effect is masked. Dr. Joule feels confident that this difficulty may 
be overcome by increasing the length of the tube. 
In a memoir on Dissociation, by M. Debray, presented to the 
Academy of Sciences, he has stated that a hydrated salt has for each 
