I02 



NATURE 



[June 3, 1897 



LETTERS TO THE EDITOR. 



{The Editor does not hold himself responsible for opinions ex- 

 pressed by his correspondents. Neither can he undertake 

 to return, or to correspond with the writers of, rejected 

 vianuscripts intended for this or any other part of NATURE. 

 No notice is taken of anonymous communications. '\ 



Effect of Change in Temperature on Phosphorescent 

 Substances. 



When a substance that possesses phosphorescent properties 

 is exposed to suitable hght vibrations, it is found to glow with a 

 certain brightness when the light is shut off. 



When such substances are exposed under similar conditions 

 for a sufficient length of time the intensity of the phosphorescent 

 light emitted reaches a maximum, which maximum is constant 

 for each substance. 



If now the temperature of the substance under observation 

 be altered, the other conditions remaining the same, it is found 

 that the maximum intensity of the light emitted varies with the 

 temperature, and that the maximum for any temperature is 

 constant for that temperature. 



The light to which the substances under observation were 

 exposed was that from a spark discharge of a Leyden jar 

 coupled up with the terminals of an induction coil. The instant 

 the spark is stopped, the intensity of the light emitted by the 

 substance is estimated by a photometer devised for the purpose. 

 The principle of the photometer consists in diminishing light 



that shines through an aperture of given area, by interposing 

 thin sheets of oil paper. A slip of glass is also interposed, of 

 the same colour as the light emitted by the phosphorescent 

 substance. As yet this photometer has only been used to com- 

 pare the maximum intensity of the light emitted from the same 

 substance at different temperatures. Now let us suppose, for 

 example, that with a specimen of calcium sulphide at - 40° C. 

 ten papers had to be interposed before the* light emitted from the 

 sulphide and that from the photometer were of equal intensity ; 

 while at 96° C. only seven papers had to be interposed. By taking 

 the reciprocal of the antilog of 10 and the reciprocal of the 

 antilog of 7, we have a rough measure of the relative intensity of 

 the light emitted at - 40° and + 96° respectively. The anti- 

 logs were plotted to scale, and from the curves thus obtained, 

 the accompanying reciprocal curves, showing how the light 

 emitted varies with the temperature, were plotted. 



Ralph Cusack. 

 Physical Laboratory, Trinity College, Dublin. 



Sinistral Screws. 

 If mechanical screws " bear" upon natural spirals, as of the 

 Gasteropoda (Nature, May 27, p. 79), it may be worth while 

 to observe that sinistral forms survive in art, as in nature. 



NO. 1440, VOL. 56] 



The silversmiths of Western India still commonly use a 

 sinistral screw of very primitive form, a pin with a wire twisted 

 round it, especially in the buckles of silver belts; things of very 

 common wear. Europeans, coming into possession of such 

 jewellery, are often sorely puzzled how to open and shut it. Yet 

 these people are as right-handed as we ; and write, as we do, 

 " w///^ the sun. " W. F. Sinclair. 



102 Cheyne Walk, Chelsea, S.W., May 28. 



Luminous Phenomena observed on Mountains. 



The following account of an occurrence somewhat similar to 

 those recently recorded in Nature maybe of interest. I will 

 give it in the observer's (Rev. W. E. Postlethwaite) own 

 words. 



"On March 5, at 9 p.m., I was crossing the Towans, wind 

 north-west ; a slight shower came on, which lasted about ten 

 minutes. During that time my hat-brim, ferrule of walking- 

 stick, finger-tips, and the edges of a book I was carrying were 

 phosphorescent, the same colour, and in some places as bright, 

 as the light emitted by a glow-worm. The light was the brightest 

 on the windward side." 



The "Towans " referred to in the foregoing account are near 

 Helston, Cornwall, on the sea coast, with an altitude ranging 

 from sea-level to 1 74 feet above, and with a general downward 

 inclination towards the north-west. 



Trewirgie, Redruth, May 29. Arthur P. Jenkin. 



The Designation of Wave-Clouds. 



In a general article on the "Photographic Observation of 

 Clouds," published in Nature of February 4 (vol. Iv. p. 332), you 

 call attention to the wave-clouds, the origin of which was first 

 explained by Helmholtz, and remark that the name of " Wogen 

 wolken " has been suggested as their designation. May I call 

 your attention to a paper I read before the Linnean Society of 

 New South Wales on August 29, 1894, in which the name 

 undulus is suggested for the ripple- or wave-clouds. The name 

 has the advantage of falling easily into line with Howard's 

 nomenclature. And some name is necessary, for out of com- 

 binations of the elementary forms stratus, cumulus, cirriis, 

 and undulus can be derived most of the diversified and ever- 

 changing cloud-groups, which never cease to delight and 

 astonish the eye and mind of man. A. H. S. Lucas. 



Newington College, Sydney, April 16. 



THE BAKERIAN LECTURE.— ON THE 

 MECHANICAL EQUIVALENT OF HEAT} 

 'T^HE purpose of this research differs essentially from 

 -*- that of any previous research on the mechanical 

 equivalent of heat. In order to diminish the loss of heat 

 by radiation, as well as to obtain the equivalent for water 

 in the neighbourhood of ordinary temperatures, the 

 ranges of temperature over which the previous dynam- 

 ical measurements have been made are greatly less 

 than the standard interval between the physically fixed 

 points of temperature to which all thermal measures are 

 referred, and so have of necessity involved the use of 

 scales, the intervals of which depend on the constancy of 

 the relative expansions of such substances as glass, 

 mercury, and air. On the other hand, in this research 

 the object has been to determine the mechanical equiva- 

 lent of the total heat necessary to raise the temperature 

 of water over the standard interval of temperature, and 

 thus to obtain directly the equivalent of the mean specific 

 heat between the freezing and boiling points. 



This undertaking is the result of the occurrence of 

 circumstances which afforded an opportunity such as 

 might not again occur. This consisted in the facilities 

 offered by the appliances which formed the original 

 equipment of the Whitworth Engineering Laboratory, in 

 1888, the more essential of these being an engine of 

 100 H.P., \yorking one of Prof. Reynolds' hydraulic 

 brakes. This brake maintains any constant moment of 



1 By Prof. Osborne Reynolds, F.R.S., and W. H. Moorby. Read 

 before the Royal Society, May 20. [Abstract.] 



