PROEESSOE BTJETSILSr AKD DE. H. E. EOSCOE’S PHOTO-CHEMICAL EESEAECHES. 923 
distance of 1 metre. These numbers cannot be supposed to be very accurate, as the 
quantity of magnesium upon which we could operate was but small, and we were there- 
fore unable to eliminate the errors arising from the phenomenon of induction as com- 
pletely as could be 'unshed. 
The combustion of magnesium constitutes so definite and simple a source of light for 
the purposes of photo-chemical measurement, that the wide distribution of this metal 
becomes desirable. The application of this metal as a source of light may even become 
of technical importance. A burning magnesium 'wire of the above thickness evolves, 
according to a measurement we have made, as much light as 74 stearine candles*, of 
which five go to the pound. If this light lasted one minute, 0‘987 metre of wire, weighing 
0T204 grm., would be bui’nt. In order to produce a light equal to 74 candles burning 
for 10 hours, whereby about 20 lbs. of stearine is consumed, 72’2 grms. of magnesium would 
be required. The magnesium -wire can be easily prepared by forcing out the metal by 
gi'eat pressure from a heated steel press ha'sdng a fine opening at bottom : this wire might 
be rolled up in coils on a spindle which could be made to revolve by clock-work, and 
thus the end of the wire, guided by passing through a groove or between rollers, could 
be continually pushed forward into a gas- or spirit-lamp flame in which it would burn. 
Y. CHE]\IICAL ACTIOA OF THE CONSTITUENT PARTS OF SOLAR LIGHT. 
The chemical action efiected by the several portions of the solar spectrum depends not 
only upon the nature of the refracting body, but also upon the thickness of the column 
of au’ through which the light has to pass before decomposition. In the following 
experiments we have employed prisms and lenses of quartz, cut by Mr. Daekee of Lam- 
beth, instead of glass prisms, which, as is well known, absorb a large portion of the 
chemically active rays. In order to render our experiments as free as possible from 
the irregularities arising fi’om variation in the atmospheric absorption, the observations 
were made quickly one after the other, so that the zenith-distance of the sun altered 
but very slightly. 
A perfectly cloudless day was chosen for these observations, and the direct sunlight 
reflected from the speculum-mirror of a Silbeemaxx’s heliostat through a narrow slit into 
our dark room. The spectrum produced by the rays passing through two quartz prisms 
and a quartz lens, fell upon a white screen which was covered 'with a solution of sulphate 
of quinine, to render the ultra-'violet rays and the accompanying dark lines visible. In 
this screen a naiTOAv slit was made through which the rays from any wished-for portion 
of the spectrum could be allowed to pass, so as to fall duectly upon the insolation-vessel 
situated at a distance of from 4 to 5 feet. A finely divided millimetre-scale was also 
placed on the screen, by means of which the distance between the Feaunhoeee’s lines 
could be accurately measured, and the portion of light employed thus exactly determined. 
In order to recognize with accuracy the various portions of the spectrum, we employed 
a map of the dark lines prepared by Mr. Stokes, which he most kindly placed at our 
* This number must only be regarded as an approximation to the truth, as the diaphragm of the photo- 
meter had to he illuminated ■with light passing a light-blue glass. 
