THE MECHANICAL ACTION OF LIGHT. 683 



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 CEOOKES, F. E. S 



SOME experiments illustrating the mechanical action of light, which 

 I have recently exhibited before the Fellows of the Royal So- 

 ciety, 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 varia- 

 tion of the force of gravitation. Experiments were thereupon instituted 

 to render the action more sensible and to eliminate sources of error.' 



My first experiments were performed with apparatus made on the 

 principle of the balance. An exceedingly fine and light arm was deli- 

 cately suspended in a glass tube by a double-pointed needle ; and at 

 the ends were affixed balls of various materials. Among the sub- 

 stances 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." B is the desicca- 

 tor, 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- 



'"On the Atomic Weight of Thallium," "Philosophical Transactions," ISTS, vol. 

 clxiii., p. 287. 



2 For a full description of this pump, with diagrams, see " Philosophical Transactions," 

 IS'ZS, vol. clxiii., p. 295. 



