employed for disinfecting purposes. When intended for preserving articles of food, or for medical or surgical uses, this crude acid must be purified, and then its color is snowy white. Rautert has succeeded in sublimating it completely in a current of superheated steam, thus readily obtaining it pure. Recrystallization from hot distilled water gives it in the form of slender needles an inch long.
| THE MECHANICAL ACTION OF LIGHT. |
By WILLIAM CROOKES, F. R. S.,
SOME experiments illustrating the mechanical action of light, which I have recently exhibited before the Fellows of the Royal Society, having attracted considerable attention, I propose to give here a description of some of the instruments which my researches have enabled me to construct. But, to render the subject more intelligible, it will be necessary to give a brief outline of the researches which I have been carrying on for the last three or four years, so that the reader may see the gradual steps which have led up to the full proof that radiation is a motive power.
The experiments were first suggested by some observations made when weighing heavy pieces of glass apparatus in a chemical balance, inclosed in an iron case from which the air could be exhausted. When the substance weighed was of a temperature higher than that of the surrounding air and the weights, there appeared to be a variation of the force of gravitation. Experiments were thereupon instituted to render the action more sensible and to eliminate sources of error.[1]
My first experiments were performed with apparatus made on the principle of the balance. An exceedingly fine and light arm was delicately suspended in a glass tube by a double-pointed needle; and at the ends were affixed balls of various materials. Among the substances thus experimented on I may mention pith, glass, charcoal, wood, ivory, cork, selenium, platinum, silver, aluminium, magnesium, and various other metals.
The most delicate apparatus for general experiment was made with a straw beam having pith masses at the end. The general appearance of the apparatus is shown in Fig. 1.
A is the tube belonging to the Sprengel pump.[2] B is the desiccator, full of glass beads moistened with sulphuric acid. C is the tube containing the straw balance with pith ends: it is drawn out to a con-
