July 15, 1920I 



NATURE 



tiy 



system of levers ; an additional lever increases the 

 weight and friction. For special research and for 

 public demonstration a still higher magnification 

 is necessary, and this I secured by the invention 

 of the Magnetic Crescograph, where a fine mag- 

 netised lever causes bv its movement 'a rotation 



Fig. 3. — Record showing the effect of carbonic acid gas. 

 Horizontal line at ihe beginning indicates balanced 

 growth. Application of carbonic ac.d g,-i» induces en- 

 hancement of growth, shown by up-curve, followed by 

 dep'ession, exhibited by down-curve. Successive dots 

 at interva's often seconds. 



of a suspended system of astatic needle with its 

 attached mirror. By graduated approach of the 

 suspended needle to the lever the magnification 

 may be continuously increased from a million to 

 ten million times. A concrete idea of the stupen- 

 dous magnification will be obtained if we imagine 



the slow pace of the proverbial snail magnified ten 

 million times. The i5-in. gun of the Queen Eliza- 

 beth throws out a shell with a muzzle-velocity of 

 2360 ft. per sec, but the crescographic snail 

 would move twenty-four times faster than the 

 cannon shot. The magnification of ten million 

 times was obtained with a single lever, but a 

 double lever will enlarge it a hundredfold-^that 

 is to say, it will give a total magnification of a 

 thousand million times. The importance of this 

 device for research in otl^r branches of science is 

 sufficiently obvious. For general purposes a mag- 

 nification of a million times is sufficient; with 

 ordinary precaution the apparatus may be ren- 

 dered free from mechanical disturbance, and the 

 zero-keeping quality of the indicating spot of light 

 is quite perfect. 



The following account of an experiment 

 in demonstration of physiological response in a 

 growing plant will be found interesting. The 

 normal growth of the plant was indicated by the 

 excursion of the ^ot of light through 6 metres 

 in lo sees. On introduction of chloroform vapour 

 to the plant-chamber there was an immediate en- 

 hancement of the rate of growth, the spot of light 

 moving three times faster. Continued action of 

 the vapour of chloroform caused, however, 

 a depression and arrest of growth ; finally, there 

 was a sudden contraction, which proved to be the 

 spasm of death. Similar effects were produced by 

 various poisons like the solution of potassium 

 cyanide. 



After this brief account of the very sensitive 

 methods for the detection and record of the effect 

 of stimulus on growth, I propose In another article 

 to describe results which will offer an explanation 

 of the tropic movements in plants Induced by 

 various stimuli of the environment. 



Isotopes and Atomic Weights. 

 By Dr. F. W. Aston. 



IN the atomic theory put forward by John Dalton 

 in 1801 the second postulate was : "Atoms of 

 the same element are similar to one another and 

 equal in weight." For more than a century this 

 was regarded by chemists and physicists alike as 

 an article of scientific faith. The only item among 

 the immense quantities of knowledge acquired 

 during that productive period which offered the 

 faintest suggestion against its validity was the 

 inexplicable mixture of order and disorder among 

 the elementary atomic weights. The general state 

 of opinion at the end of last century may be 

 gathered from the two following quotations from 

 Sir William Ramsay's address to the British Asso- 

 ciation at Toronto in 1897 : — 



There have been almost innumerable attempts to 

 reduce the differences between atomic weights to 

 regularity by contriving some formula which will 

 express the numbers which represent the atomic 

 weights with all their irregularities. Needless to say, 

 such attempts have in no case been successful. Ap- 



NO. 2646. VOL. 105] 



parent success is always attained at the expense of 

 accuracy, and the numbers reproduced are not those 

 accepted as the true atomic weights. Such attempts, 

 in my opinion, are futile. Still, the human mind does 

 not rest contented in merely chronicling such an 

 irregularity ; it strives to understand why such an 

 irregularity should exist. . . . The idea . . . has been 

 advanced by Prof. Schutzenburger, and later by Mr. 

 Crookes, that what we term the atomic weight of an 

 element is a mean ; that when we say the atomic 

 weight of oxygen is 16, we merely state that the 

 average atomic weight is 16 ; and it is not incon- 

 ceivable that a certain number of molecules have a 

 weight somewhat higher than 32, while a certain 

 number have a lower weight. 



This idea was placed on an altogether different 

 footing some ten years later by the work of Sir 

 Ernest Rutherford and his colleagues on radio- 

 active transformations. The results of these led 

 inevitably to the conclusion that there must exist 

 elements which have chemical properties identical 

 for all practical purposes, but the atoms of 



