50 



NATURE 



[September i8, igig 



on the rate of chemical change which — in conjunc- 

 tion with those of Berthelot in France and those 

 of Guldberg in Norway — were to establish on a 

 quantitative basis Berthollet's law of mass action. 

 In the interpretation of his results Harcourt was 

 associated with William Esson, whose special 

 mission in Oxford seemed to be — -as the writer 

 knew him — to illuminate mathematically the 

 obscure records of chemical velocities. Harcourt 

 and Esson first studied the reaction between oxalic 

 acid and potassium permanganate in acid solu- 

 tion. They found that the rate of change varied 

 with the amount of manganous sulphate formed, 

 and that the reaction was probably nil in the com- 

 plete absence of manganous salt ; but, once 

 started, the velocity would increase to a maximum 

 and then slack off- — the curve reprcsenving the 

 course of the change having a point of contrary 

 flexure. They liken their curves to those obtained 

 by Bunsen and Roscoe in the course of photo- 

 chemical induction — thus suggesting that the 

 " inductive period " in the union of hydrogen and 

 chlorine was due to the action of another sub- 

 stance, a suggestion which finally was proved to 

 be correct. Seeking for a less complicated re- 

 action, Harcourt found that in dilute solutions 

 hydrogen peroxide decomposed hydrogen iodide 

 with velocities that could be easily followed, and 

 the amount of change could be accurately ascer- 

 tained. The method of carrying out the experi- 

 ment in a stream of carbon dioxide, and the device 

 by which the iodine liberated was reconverted into 

 iodide by the successive additions of exactly equal 

 drops of concentrated thiosulphate, show Har- 

 court at his best as an experimenter. The time- 

 intervals between the successive appearances of 

 the iodine proved that the velocity of the change 

 varied directly with the quantities of each of the 

 reacting substances- — when the other conditions 

 were kept constant. The rates found, however, 

 do not prove that the change is necessarily a tri- 

 molecular one as Harcourt supposed : — 



H20o-l-2HI = 2H20-|-l2. 



The change most probably takes place in two 

 stages, each of which is di-molecular ; but, one 

 stage being much faster than the other, the 

 observed rates follow the simple law. 



In studying the effect of temperature on the 

 rate of this reaction Harcourt and Esson arrived 

 at a zero of chemical action in wonderful agree- 

 ment with the absolute zero calculated from 

 physical data. 



Harcourt was so strongly convinced that 

 chemical change followed mechanical laws that 

 his laboratory became a centre whpre the experi- 

 ments of Bunsen and his school on "chemical 

 induction" and " sprungweise " explosions were 

 repeated and criticised. 



Harcourt's work as one of the metropolitan 

 gas referees led him to take up the investigation 

 of sulphur impurities in coal gas and to design a 

 new staindard of light — the Pentane standard. 

 His method of converting carbon disulphide into 

 the easily removed hydrogen sulphide has only, 

 NO. 2603, VOL. Ip4] 



recently been adopted on a large scale in the 

 South Metropolitan Gas Works through the 

 energy and skill of Dr. Cairpenter, but its success 

 seems assured. The lo-candle Pentane lamp is 

 not only the official British standard, but also as 

 a practical unit is not approached by the German 

 amyl acetate lamp. 



Few men have been so completely happy in 

 their work, or lived so much in the lives of their 

 students. The writer, whose good fortune it was 

 to fall under his influence at Oxford, has to 

 acknowledge that he owes his career to Harcourt's 

 affectionate interest and to his example. 



V\.' H. B. D. 



DR. CHARLES A. MERCIER. 



IT is not possible to give, in a few words, more 

 than a mere indication of the value of the 

 late Dr. Mercier's scientific work. With a rare, 

 natural capacity for clear thinking, as well as for 

 acquiring and retaining knowledge, he was a 

 master of luminous and logical expression in 

 speech and writing. These qualities mform all 

 his many and various works, whether of purely 

 scientific or mainly literary nature. Some of 

 his books deal with the practical aspects of his 

 professional speciality — insanity — such as the 

 management of asylums, instructions to nurses, 

 etc.; and others, hke his essays on "Tempera- 

 ments," treat of psychology and conduct for the 

 general reader, as well as for the expert. But 

 the main works, on which his reputation will rest, 

 are scientific studies of the nervous system in 

 health and disease, and include specially the whole 

 subject of the causes, conditions, and expressions 

 of mental action, normal and morbid. He was, 

 as a student, much influenced by the writings of 

 Herbert Spencer, and by the personal teaching cf 

 Dr. Hughlings Jackson, and their influence is seen 

 in many of his works; but no '.ess evident is his 

 originality of thought, as especially seen in 

 some of his more recent publications, which indi- 

 cate strongly a marked change of attitude towards 

 the so-called " Lamarckian " doctrine of biological 

 evolution to which at first he strongly adhered. 

 Like his two chief teachers, he made much use 

 of the deductive step in reasoning-, but he did not 

 often fail to verify his conclusions by further 

 evidence before adopting them. Judged from the 

 scientific viewpoint, some of his most important 

 •works — e.g. on the "Nervous System and the 

 Mind," on "Psychology, Normal and Morbid," 

 and those dealing with insanity, may be con- 

 sidered as holding the highest rank among books 

 of this kind, and as at least equal in value (though 

 greatly differing in certain respects) to the now 

 classical works of the late Dr Henry Maudsley, 

 his illustrious senior and contemporary, on the 

 "Physiologv and Pathology of Mind," "From 

 Organic to Human," "Body and Mind," etc. 



Dr. Mercier's work, "The New Logic," has 

 not pleased some professorial logicians, but, as 

 a handbook of logical reasoning for scientific 



