526 PROCEEDINGS OF THE AMERICAN ACADEMY 



for the final decisive experiments. During the following winter, with 

 the co-operation of T. W. Richards, numerous determinations were 

 made, in which an accurately weighed quantity of hydrogen was 

 burned, and the weight of water which was formed determined. The 

 experimental difficulties involved were great although they were in 

 kind quite different from those which the work upon antimony pre- 

 sented. These difficulties were at last overcome, and the wonderfully 

 close agreement between sixteen successive determinations, made 

 with hydrogen prepared in three different ways, showed how perfectly 

 all sources of accidental error had been eliminated. 



One source of constant error had been overlooked, however, which 

 affected not only these determinations but all the results which had ever 

 been obtained by the classic method of Regnault. Agamennone, and 

 afterward Lord Rayleigh, discovered that the volume of a glass vessel 

 was sensibly diminished when the air within it wa;^ exhausted, so that 

 the tare of the globe in which the hydrogen had been weighed had 

 been incorrectly determined on account of the diminished volume of 

 the air displaced. The amount of this error could easily be deter- 

 mined, and in a second paper the necessary small correction was ap- 

 plied. In order to avoid this change of volume, he devised an ingen- 

 ious method for determining the tare of the globe without exhausting 

 it ; and this method was the subject of his last communication to the 

 Academy, in June, 1889. 



As an investigator Professor Cooke was clear in thought, perse- 

 vering amid difficulties, fertile in expedients, impatient of dogma, and 

 to the end he retained the keen curiosity and enthusiasm of his earlier 

 days. 



ADDKESS OF AUGUSTUS LOWELL. 



Tt is always interesting to trace the impulse which had determined 

 a man's life work. It is peculiarly so to me in the case of Professor 

 Cooke, because he himself was wont to attribute it in a large measure 

 to the impression made upon him as a boy by the lectures of Professor 

 Silliman, before the Lowell Institute. These lectures opened a new 

 horizon. He was intensely interested, repeating the experiments he 

 had witnessed with such imperfect appliances as he was able to pro- 

 cure, and from that moment there was no hesitation as to his future. 

 His life was to be devoted to the study of Chemistry. 



There was nothing in his birth or surroundings to indicate such a 

 career. His father was a successful lawyer ; his maternal ancestors 



