TRANSACTIONS OF THE SECTIONS. 59 
and with protosulphate of iron, for the purpose of ascertaining the exact quantity of 
bichromate of potash corresponding to a certain weight of iron. The mean of all his 
experiments gives 100 of iron to 88°75 of bichromate. In the analysis of those varie- 
ties of ironstone which contain an admixture of peroxide of iron, Dr. Penny recom- 
mends the employment of sulphite of soda, by which the peroxide is speedily reduced ~ 
to the minimum state of oxidation, and then the bichromate may be applied as before 
indicated. The only precaution to be observed is to use the sulphite of soda in suffi- 
cient quantity, and to take care to expel the excess of sulphurous acid by brisk ebul- 
lition, Dr. Penny likewise noticed several objections, which appear to militate 
against the accuracy of this method of analysis. He showed that no error jcould 
arise, during the performance of the experiment, from the oxidation of the proto- 
compound of iron in the hydrochloric acid solution by the oxygen of the air, as no 
appreciable change can be detected even after the lapse of several hours. And he 
also satisfied himself that hydrochloric acid exerts very little influence upon dilute 
solutions of the chromates, so that no inaccuracy could be produced by this action. 
Im conclusion, Dr. Penny remarked, that his paper might be much extended by 
showing the application of this process to the analysis of other ores of iron, as well as 
its availability for the examination of alum liquors and copperas liquors, and other 
products of the arts; but such applications can easily be made by any one ordinarily 
acquainted with practical chemistry. 
. Phophorescence of Potassium. By WiLuiaM Petrie. 
It was not accident that led Mr, Petrie to observe the fact of the phosphorescence 
of potassium, but while speculating on the consequences of the dynamical theory of 
heat he was led to the conclusion that cold potassium ought to be found /uminous, and 
further, that it ought to be only about a tenth part as luminous as phosphorus. 
On testing this experimentally, as soon as opportunity allowed, with the precautions 
for sensitive vision which the anticipated feebleness of the light indicated to be neces- 
sary, the result was, that on dividing a bit of potassium (which was quite dry, being 
protected only by a coating of bees’-wax) the halves showed two distinctly luminous 
sections, the light being about a tenth of that from a similar surface of phosphorus. 
The light diminished naturally as a protecting coat of oxide was formed, but remained 
just perceptible to the most sensitive sight as long as half an hour, 
The outline of the considerations which led theoretically to this fact may be worthy 
of a brief notice. Light from electricity or from combustion is due to the intense 
heat produced by the electricity or by the chemical combination ; in other words, the 
transfer of electric power in the one case, or, in the other, the violent impulsion of the 
atoms of carbon and oxygen, by affinity, from their natural atomic distances towards 
each other, produces atomic motions, which (having nothing to absorb the dynamic 
force inyested in them) must continue as a state of intense vibration, causing the phe- 
nomena of heat. Some of the vibrations communicated to the surrounding ether are 
of a transverse order, and some of these are of that intensity which affects the eye with 
the sensation of light. 
But, such being the cause of light, how can phosphorus emit light when not warm? 
Because the few indiyidual atoms which are at avy moment in process of oxidation 
must necessarily be at the instant thrown into the full state of vibration due to the 
force of affinity acting during their transit onward from the distance ordinarily sepa- 
rating them from those of the oxygen, and this vibration must continue until it is 
gradually expended in imparting vibrations of radiant heat and light to the surround- 
ing ether, besides gradually dividing its force into slighter vibrations in all the adja- 
cent atoms of its own: substance, that is, raising its temperature in a minute degree. 
The heat of so few atoms vibrating intensely is scarcely perceptible, while another 
substance may have αἰΐ its atoms in a state of general vibration such as to be much 
hotter, and yet not one of them vibrating with sufficient intensity to produce light. 
Now it is clear that the greater the number of vibrations which each of the few 
combining atoms can make and communicate to the surrounding ether, before the 
atom loses its motion, by gradually imparting it to all the surrounding atoms of the 
substance, so much the more light will each of the combining atoms have produced, 
on the whole. 
In the case of phosphorus, a Jarge portion of the motion of any vibrating oxidized 
