^03 



NA TURK 



[May 13, looQ 



Mr. McLennan has done his arithmetic correctly, but 

 the calculation is really extremely simple, as thus : — • 



The moon's mass is i/8oth that of the earth, which is 

 6x10-' tons. At the moon's distance, which is 60 earth's 

 radii, terrestrial gravity is reduced to i /3600th of its value 

 at the earth's surface. Consequently, the weight of the 

 moon, i.e. the earth's gravitative pull on it, is equal to 

 the ordinary commercial weight of 



1/3600 X i/So X 6 X 10-' tons. 



An alternative, but not a supplementary, way of doing 

 the sum is to say that the moon revolves through the 

 angle 2-ir in, say, 27J days, and that therefore the centri- 

 petal force necessary to hold it in is 



tons X 



\27Ji days/ 



60 X 4000 miles. 



80 \27Ji days. 



The acceleration part of this is about 13,000 miles per 

 day per day, which is the same as 32/3600 feet per second 

 per second ; and this is obviously in accordance with the 

 law of inverse square. Oliver Lodge. 



A Direct Estimate of the Miniinum Age of Thorianite. 



I WISH to record an experiment lately made which 

 affords more direct proof of the great antiquity of radio- 

 active minerals than anything previously attempted. 



The principle of the method is to determine (i) the 

 total accumulated helium in the material ; (2} the rate of 

 formation in the same material at present. 



A sample of thorianite was found to contain 9 c.c. 

 helium per gram. 



Four hundred grams of this thorianite was got into 

 solution, and all traces of helium boiled out with scrupulous 

 care. After it had been allowed to stand for seven weeks, 

 the solution was boiled out again. A little nitrogen and 

 nitric oxide were evolved. These were removed by 

 charcoal cooled in liquid air, and the residual helium 

 collected in the capillary of a McLeod gauge. D, could 

 be seen in the spectrum, but the volume of the gas was 

 too small to measure. It was certainly less than 

 2XI0-' c.c, perhaps much less. 



Thus the annual rate of production of helium per gram 

 is less than 37x10-' c.c. The q c.c. initially present 

 cannot, therefore, have accumulated in a less time than 

 240 million years. Experiments on a larger scale, which 

 are in progress, will probably lead to an extension of this 

 estimate. 



Prof. Joly, in his interesting book, " Radio-activity and 

 Geology," has brought various objections against the 

 radio-active method of measuring geological age. These 

 will require to be carefully weighed. I may remark, how- 

 ever, that in the present case no uncertainty arises on 

 the ground that the substance may formerly have con- 

 tained much more uranium or thorium than at present, 

 for thorianite consists almost entirely of the oxides of these 

 elements. ' R. J. Strutt. 



Imperial College of Science, May 11. 



Sense of Smell in Flies. 



By far the most efficient of fly-destroyers with which 

 I am acquainted is a dilute solution of formaldehyde. 

 If two teaspoonfuls of formalin {40 per cent, formaldehyde) 

 be added to a soup-plate filled with water, flies go to it, 

 one after the other, to drink, especially in the early after- 

 noon. Some die in the water ; many fall in the immediate 

 neighbourhood of the plate ; others succumb on window- 

 sill or floor. As the result of leaving a single plateful of 

 the solution on the kitchen table (I am writing in the 

 south of France) hundreds of dead flies are each day 

 swept up from the floor. Formalin water is free from 

 the gruesome associations of fly-papers and other traps 

 which hold their struggling victims. It may even be 

 turned to ornamental uses. A wire cage placed in the 

 centre of the dish may be crowned with flowers, which 

 flourish equally as well, with some slight but interesting 

 changes in tint, in dilute formalin as in pure water. The 

 solution neither attracts nor repels flies. Two similar 

 dishes placed side by side, the one containing pure water 

 and the other formalin, are visited, so far as one can judge, 

 with equal frequency. It is somewhat strange that so 

 NO. 2063, VOL. 80] 



small a dose proves fatal when taken into the fly's 

 aliinentary canal. I find that, to free a room from flies 

 by vaporising formalin, the air must be rendered quite 

 irrespirable by a human being. The room needs to be 

 amply ventilated before one ventures into it. 



The interest which attaches to this observation, that 

 flies will drink a solution of formadehyde, lies in the proof 

 which it affords that the mechanism of their sense of 

 smell is similar to our own. No volatile body the density 

 of which is not greater than that of air is a stimulant 

 of our olfactory membrane. Formaldehyde, H,COH, has 

 a density of 15 only. Playing in paradoxes, one might 

 say that it undoubtedly has a malignant odour, but we 

 cannot smell it. If the nose be placed close to a vessel 

 containing a dilute solution of formalin a scent is' recog- 

 nisable, but this I take to be due (a chemist will correct 

 me) to impurities present in the commercial product. Yet 

 I find that when I sit within a yard of it my eyes begin 

 to smart. In this respect, however, I am, I know, 

 exceptionally sensitive. I cannot dissect specimens pre- 

 served in formalin until after they have been long soaked 

 in water and spirit frequently changed. Once, when con- 

 ducting a viva voce examination with the aid of formalin 

 preparations, I developed so acute and painful, although 

 happily transient, an attack of conjunctivitis as made it 

 impossible for me to attend the examinees' meeting. The 

 fact that so deleterious a volatile body as formadehyde does 

 not appeal to our sense of smell would seem to confirm 

 the only theory of the physics of olfaction at present 

 plausible, though far from comprehensible, namely, that 

 which attributes to the hairs of the cells of the olfactory 

 membrane the capacity of responding to the alterations in 

 the vibration frequency or amplitude of molecules of air 

 which are caused by the presence amongst them of heavier 

 molecules. Alex. Hill. 



Mentone. 



The Production of Radium from Uranium. 



Experiments on which I have been engaged for the 

 past si.x years have until now failed to establish the pro- 

 duction of radium from uranium. With carefully purified 

 uranium salts, using considerable quantities, the growth 

 is too small to be detected for the first two or three 

 years, and is less than i/io,oooth of what would occur 

 if a direct change of uranium into radium took place. 

 With commercial uranyl nitrate, on the other hand, 

 purified from radium by precipitating barium sulphate in 

 the solution, a distinct though small production of radium 

 was observed in 1905, and subsequently confirmed (Phil. 

 Mag., October, 190S, 632). This is explained by the 

 existence of an intermediate parent of radium in the series 

 with a very long period of life, which has been found by 

 Boltwood and by Rutherford in preparations of actinium, 

 and recently isolated by the former from uranium minerals, 

 and called " ionium." 



I have now been able to establish the production of 

 radium in all the solutions of very carefully purified uranyl 

 nitrate prepared by Mr. T. D. Mackenzie and myself (Phil. 

 Mag., August, 1907, 272). Three separate solutions con- 

 taining 255, 408, and 278 grams of uranium (element), and 

 of age since purification at the present time respectively 

 353, 240, and 2-73 years, have been kept under observa. 

 tion. The method of testing has been much improved 

 since formerly, and the error of a single determination as 

 now carried out probably does not exceed 10-'^ gram of 

 radium. The quantity of radium in the oldest solution is 

 now about 4X10-" gram, which is nearly twice as great 

 as initially. During the past year five measurements of 

 the quantity of radium in this solution have been made, 

 and they show that, within the stated limit of error of 

 observation, the production of radium has proceeded during 

 that period proj^ortionally to the square oj the time. The 

 tests on the other solution show that in them also the 

 production of radium is now proceeding according to the 

 same law and at a similar rate. 



That the initial rate of production of radium from 

 uranium should vary according to the square of the time 

 was deduced mathematically by Rutherford (Jahr. Rad., 

 iqoS, v., 164) on the assumption that there was only one 

 intermediate substance of period of life long compared with 



