July 15, 1897] 



NATURE 



259 



The results of all ihis labour are shown by the diagram (p. 

 258), and also by the following table. 



^B terminations over any appreciable temperature range, and it is 

 ^H- impossible, therefore, to apply the " rate of change" com- 

 ^m parison to their results. Their thermometry, however, was 

 ■ directly based on the International Standard, hence it is probable 

 B that we now have thee separate determinations , of the 

 B " mechanical equivalent " in which the temperature scale is 

 B identical. 

 fT It appears to me that the data now at our disposal justify the 



following conclusions : — 



(a) That the rate of change in the capacity for heat of- water 



from 10' to 25° C. may be considered as known with sufficient 



accuracy for present purposes. 



(/') That the persistent difference in the capacity for heat of 



water, when determined by mechanical and electrical methods, 



indicates a possibility that there is some error in one of the 



electrical units. ^ 



One further matter deserves attention, concerning which there 



has hitherto existed considerable uncertainty, viz. the value of 



the ratio 



mean thermal unit 

 thermal unit at t' 



The value of this ratio is of great importance, as in the 

 absence of exact information on this point we are unable to 

 utilise the results of many notable experiments, such as those 

 performed with Bunsen's calorimeter. 



Until recently Regnault's value ( 1*005 '" terms of the thermal 

 unit 0° to 1°) has been universally adopted. 



Some experiments performed by me in 1894, on the latent 

 heat of evaporation of water, led to the conclusion that " the 

 value of the ' mean thermal unit ' is practically identical with 

 that at 15^ C." (YVans. Kay. Soc, 1895, P- 32o)- 



On the publication of this statement, Dr. Joly performed 

 some experiments, from which we obtain the value •9962 in 

 terms of the thermal unit at 15 C. {Phil. Mag., November 

 1S95, P- 440). assuming Rowland's uncorrected values from 

 o' to 15°.- 



In the Report of the Electrical Standards Committee, 1896, 

 Mr. Shaw gives the results of a recalculation of Regnault's 

 numbers, in which, assuming Rowland's uncorrected values, he 

 obtains i*ooi6 in terms of the thermal unit at 10". 



The enumeration of these facts indicates the extent of our 

 uncertainty, and here again recent investigations lead us oh to 

 ^rmer ground. 

 ^^ In the Bakerian lecture delivered by Prof. Osborne Reynolds, 

 'n May 20, 1897, he communicated the results of an investigation 

 by Mr. Moorby and himself into the value of the " mean thermal 

 unit." Their results are of peculiar importance because they 

 are practically independent of temperature measurements, and 

 also on account of the large scale on which they were conducted. 

 Their conclusion is " 776 '94 me^chanical units at Manchester." 

 I take this as equal to 777*07 at Greenwich = 4184 x 10* ergs. 

 This is about equal to Rowland's corrected value at 19^, or, ex- 

 pressing it in terms of Rowland's corrected value at 15°, we get 

 •9988, which is sufficiently near to unity to justify my prediction 

 as to the practical equahty of the two units. 



I trust that the above short summary will suffice to show that 

 great advances have recently been made, and I venture to 

 express a hope that the importance of extreme accuracy with 

 regard to thermal measurements may, in the future, be more 

 generally recognised than, I believe, has been the case in the 

 past. E. H. Orikfiths. 



' For example, an error of i in 1000 in the electro-chemical-ec|uivalent of 

 silver would account fornearly the whole of our present discrepancies. 



- I find that if we take Rowland's corrected values, this number approxi- 

 mates to "9975. 



NO. 1446. VOL. 56] 



THE ACTION OF LIGHT ON DIASTASE. 



"T^HE influence of the different rays of the solar spectrum 

 ■^ upon the various phenomena of vegetable life has been 

 shown by many observers to be not at all uniform. Speaking 

 broadly, the rays lying to the left of the green, often collectively 

 termed those of the red end, have been found to be most actively 

 concerned with the metabolic processes. They are the ones on 

 which generally the working of the chlorophyll apparatus 

 depends, and in their absence no construction of carbohydrates 

 from the carbonic anhydride of the air takes place. The rays 

 beyond the green to the right, including also the ultra-violet 

 ones, have, on the other hand, been shown to play but a small 

 part in such constructive processes, but to be those on which 

 the phenomena of heliotropism and other interferences with 

 growth depend. They are broadly associated, therefore, with 

 the physical rather than the chemical processes. 



In recent years the influence of the blue, violet and ultra- 

 violet rays has been found to be deleterious to vegetable 

 protoplasm, exposure to them destroying many micro-organisms. 

 They have further been .shown to share, though to a small extent^ 

 only, the power of assisting the chlorophyll apparatus. Some 

 time ago a research of considerable importance was conducted 

 by Messrs. Brown and Morris, from which it appeared that the 

 amount of diastase obtainable from foliage leaves varies con- 

 siderably at different periods of the day, being greater after 

 darkness, and diminishing after exposure to light. An investi- 

 gation of the action of the different rays of the spectrum on 

 diastase has recently been carried out by Prof. Reynolds Green, 

 which .shows that its separate regions possess radically different 

 powers, and that while some rays are beneficial and aid in the 

 secretion of the enzyme, others are as distinctly deleterious and, 

 indeed, affect the diastase in the same way as those of the blue 

 end do the micro-organisms already spoken of. 



By the use of appropriate screens the spectrum was divided 

 into seven bands, the infra-red, the red, including the rays of 

 wave-length TZOfxix to 640/1/^ ; the orange, ranging from 640ju/it 

 to 585|U/i ; the 'green, from S^Sl^l^ to 500/iju ; the blue, from 

 500yu/t to 430jLijU ; the violet, including the visible rays beyond 

 wave-length 430/i;u ; and the ultra-violet. 



Solutions of diastase, prepared from malt extract, and from the 

 leaves of Phaseolus vulgaris, as well as from human saliva, 

 were exposed to strong illumination, either that of the sun, or 

 of a strong naked electric arc-lamp, for several hours, and after 

 such exposure their hydrolytic power was tested side by side 

 with that of concrol solutions that had not been illuminated. By 

 the use of the screens already mentioned, the effects of these 

 several regions of the spectrum were ascertained. 



Ofthe infra-red, red,orange and blue regions the rays were found 

 to^ave a distinctly beneficial effect upon the manufacture of the 

 enzyme. The effect was, however, much the greatest in the red 

 region, these rays, when allowed to act for ten to twelve hours, 

 increasing the amount of diastase by one-half. A second 

 maximum was obtained in the blue, while the rays of the other 

 regions were intermediate in power. 



The violet and ultra-violet rays were ascertained to exert a 

 destructive action on the enzyme, an expo^re of ten to twelve 

 hours to the latter especially, often resulting in the destruction of 

 60 per cent, of the diastase. The change set up in the solution 

 was found to be a continuous and progressive one, the extract 

 becoming weaker after removal from the action of the light until 

 the diastasic power entirely disappeared. 



This effect was found to follow also a strong illumination of 

 the living leaf. Leaves of Phaseohis were exposed to sunlight and 

 the electric arc respectively, half of each being carefully shaded 

 by a cover of blackened paper. After the illumination, weighed 

 quantities of the shaded and unshaded sides were made to act 

 under the same conditions upon a solution of starch, when the 

 hydrolytic power of the unscreened portions was found to have 

 been very materially injured. 



The difference in the action of the various rays suggests a 

 modification of the view that the chemical action of light in 

 vegetable metabolism is particularly a property of the red end 

 of the spectrum. This is borne out further by some experi- 

 ments recently published by Laurent, who has found that the 

 blue and violet rays are especially active in the construction of 

 nitrogen-containing compounds in the plant. All the rays 

 appear to be able to bring about chemical effects, the differences 

 depending upon the materials taking part in the reactions, rather 

 than upon any radical differences in the nature of the light. 



