5 i8 HISTORY OF SCIENCE. 



planations which have as yet been advanced to account for them leave 

 so many points to be cleared up that these explanations cannot yet be 

 considered as satisfactory. For this reason, and because these matters 

 are still sub judiceor subjects of controversy among physicists, we shall 

 refer to them as little as possible, but confine ourselves to the facts, in 

 which our readers will hardly fail to recognize the great experimental 

 skill and ingenuity displayed in the conduct of this refined and delicate 

 investigation. 



After Mr. Crookes had discovered thallium and examined some of 

 its properties, he undertook to determine with the utmost possible 

 accuracy the atomic weight of the new element. Such determinations, 

 simple as they are in principle, require in practice a multitude of 

 minute precautions extended over a prolonged series of experiments. 

 Thus when substances are to be weighed with scientific accuracy, it 

 is necessary to correct the apparent weight by a calculation for the 

 effect of the surrounding air in buoying up both the weights and the 

 body to be weighed. This requires observations of the barometer, 

 thermometer, and hygrometer to be made at the time of weighing, in 

 order that the weight of a given volume of air may be estimated. The 

 volumes of the weights and of the body to be weighed must also 

 be known. Besides saving the labour in these observations and 

 calculations, there are other great advantages in having a balance 

 working in a vacuum. The weight of a vessel is liable to change from 

 the deposit on its surface of a film of moisture. Further, it is impos- 

 sible to find, by weighing in the air, the weight of a body having a 

 temperature different from that of the air. Thus if a vessel warmer 

 than the surrounding air were placed in the pan of a balance, the 

 vessel would seem to be lighter than its true apparent weight on ac- 

 count of an ascending current of air which would be produced by the 

 excess of temperature. Now, it was precisely in order that he might 

 be able to weigh in a warm condition certain things required for his 

 thallium determinations, that Mr. Crookes had constructed a delicate 

 balance enclosed in an air-tight metallic case with a glass front. From 

 this case the air could be exhausted. In this apparatus Mr. Crookes 

 observed some curious results, particularly that a body appeared to 

 be lighter when warm, notwithstanding the vacuum. He then made 

 special experiments to render the action, if possible, more evident, and 

 to clear away certain sources of error. He was led to devise apparatus 

 of a more and more delicate kind, by which he was at length enabled 

 to demonstrate at will, that radiant heat and light acting on bodies' 

 within vacuous vessels were capable of producing motion in visible 

 masses. 



The power of light and of heat to impart motion had been previ- 

 ously the subject of but few experiments. Mr. A. Bennett in 1792 

 had tried in vain to make a delicately suspended beam turn under the 

 influence of light concentrated on one of its arms. One or two phy- 



