36 REPORT—1843. 
to the time of Niepce and Daguerre, have shown many peculiarities in the action of 
this agent. But since that period the list has been wonderfully increased by the re- 
searches of Wollaston, of Davy, of Fox Talbot, and, above all, by the extraordinary 
discoveries of Sir John Herschel. We are now acquainted with combinations of 
silver, of gold, of mercury, of iron, and many non-metallic bodies, which are speedily 
changed under the sun’s influence, and which are sufficiently sensitive to be used as 
photographic agents. The author has been successful in adding platinum to the list ; 
which metal gives considerable promise of utility in the art. He has also been suc- 
cessful in producing photographic images on the salts of manganese, of tin, of anti- 
mony, of lead, of cobalt, and of arsenic. He has produced pictures with chlorine, 
iodine, and bromine vapours received upon the surfaces of a great variety of metals, 
and even on wood and on leather; and many of the alkaline and earthy salts have 
given evidence of this extraordinary property of the sun’s rays. The author con- 
tended, that from the extensive series of results which he had obtained, he was fully 
warranted in expressing it as his opinion, that all bodies were constantly, under the 
influence of the solar emanations, undergoing a change of state; that, indeed, photo- 
graphic images were always formed, cn whatever body a shadow fell: we were only 
ignorant of the reagents by which these images could be called forth; but we were 
rapidly arriving at the knowledge we desired. 

On a new Method of testing the Hygrometric Formula usually applied to Ob- 
servations made with a wet and dry Thermometer. By J. Avsoun, M.D. 
Some years ago Professor Apjohn communicated to the Royal Irish Academy a for- 
mula for inferring what is called the dew-point, from observations of the wet and dry 
thermometers alone, and he considered the accuracy of his formula as fully established 
by three experimental tests, which he explained in a memoir read to the same learned 
body. ‘But finding,” he says, “that some still doubted, I thought it might be as 
well to give publicity to a fourth method of verification which I have lately em- 
ployed, and which has in the fullest manner sustained the general conclusion to which 
the previous test experiments had conducted. It is well known that if unity repre- 
sents the specific gravity of dry atmospherical air at the temperature of 60°, and 
under a pressure of 30°, that its specific gravity at the temperature ¢", and under the 
pressure f" will be represented by the expression 
Ti 
461 +7" 30 
Now if we suppose the air at 2 to be saturated with moisture whose elastic force is 
P 5 fi 
", the specific gravity of the aqueous vapour will be ——, x 4 X_ ‘625, the lat- 
f'; P gravity q pour wi be er 30 625, 
ter factor being the density of vapour in relation to air, having the same temperature 
with it, and existing under the same pressure. The air, however, being not at its 
dew-point, but at some higher temperature ¢, in order to obtain the specific gravity of 
the vapour at this latter temperature, the expression already got must be multiplied 
HM i 
by ene by which it will become - i +x f X *625. This, then, is the exact 
specific gravity of aqueous vapour in air, whose temperature is ¢, and whose dew-point 
is t!'. And multiplying this by *31 v, ‘31 being the weight of one cubic inch of atmo- 
spheric air, and v the volume, or number of cubic inches of air, 
OMRON Liter 1a, , __ 3°3647 f" 
a1 ne * 30% 625 x oP aE! 8 
of aqueous vapour whose maximum tension is f", and existing in v cubic inches of 
air whose temperature is ¢. Let us suppose this quantity to be determined by expe- 
riment, and to be represented by w. Then 
33647, f" 1 w . 46142 
Ie KOM ond l= FX Seay 
Knowing therefore w, which may be got very accurately by experiment, we can cal- 
—— ! — 
culate /”. But, by the hygrometric formula, f" =f! — : _ xf a : this latter 

x v is the weight of v cubic inches 


