June 29, 1905] 



NA TURE 



199 



logether the original data in order to compare the light 

 pressure upon a vane of clear glass witti that upon a 

 silvered surface. 



The experiment may be here recalled. A torsion balance 

 carrying a thin vertical glass vane, 14X10x0-1 mm., 

 ^ih'ered on one side, was suspended in a bell jar, and the 

 air was pumped out until the pressure was about 40 mm. 

 of mercury. A beam of light was thrown upon this vane 

 at a definite distance from the rotation axis, and by turns 

 on each side of it. The deflections were read by a tele- 

 .scope and scale. A Nernst lamp was used as a source, 

 the intensity being given by a precision wattmeter. The 

 balance was then turned through 180° by the rotation 

 of the external control magnet, and readings were again 

 tal<en. The mean was proportional to the pressure of the 

 incident and reflected beam. The mean reflection coefficient 

 of air-silver and air-glass-silver for the radiation used 

 has been found to be 85 per cent. The pressure, according 

 to Maxwell's theory, should therefore be 185 times that 

 due to the incident beam. The throw obtained (contain- 

 ing certainly less than i per cent, of gas action) was 228 

 divisions. Hence the pressure of the standard beam upon 

 a black surface would be 22-8-f- 1-85 or 12-4. 



The balance was then taken from the bell jar, the silver 

 removed from the vane, and the glass surface cleaned. 

 The balance was then replaced, and the air pumped out as 

 before. The deflections were small, only about 2 mm., 

 and therefore could not be read to a greater accuracy than 

 5 per cent. The throw obtained for standard lamp was 

 2-1 divisions (the mean of forty observations at four 

 different air pressures). 



The normal reflection coefficient of glass (^ = 1-52) for 

 this kind of radiation is 4-1 per cent. The amount re- 

 flected from the two surfaces is approximately 8-2 per 

 cent. Hence the energy per unit volume in front of the 

 glass is about 1-082 times that of the incident beam, and 

 that behind the vane (since the absorption is negligible) 

 is o-qi8 times that of the incident beam. The former 

 quantity is greater than the latter by 16-4 per cent, of 

 the energy of the incident beam. .\ssuming that the 

 pressures on the front and back surfaces of the glass are 

 proportional to the energies per unit volume, the pressure 

 of the standard beam upon a black surface would be 

 2-1-^0-164 or 12-7. The agreement between this result 

 and the similar result obtained from the silvered surface 

 shows that light passing through a plate of glass exerts 

 pressures upon the surfaces equal to the difference between 

 the energies per unit volume in front of and behind these 

 surfaces. Gordon F. Hull. 



Dartmouth College. Hanover, N.H., U.S.A. 



The Habits of Testacella. 



Until reading Mr. Latter 's letter in this week's Nature 

 I was unaware that it was not a matter of common know- 

 ledge that Testacella appears on the surface during heavy 

 rains. My garden is liable to be flooded, as also, un- 

 happily, is much of this neighbourhood, in spring and 

 late autumn. After the water has stood for a few days 

 the ground is covered by hundreds of these slugs, which 

 leave their burrows and trv to find dry quarters. Thev 

 can. survive, however, a week's immersion. In June, 1903. 

 when much of the Thames valley was flooded, I collected 

 a number of these slugs for various malacological friends. 

 In normal circumstances they live at such a depth as never 

 to be unearthed during garden operations. 



Eton College, Windsor. M. D. Hill. 



NATURE AND MAN. 



PROl". LANKESTER in his Romanes lecture bcfjan 

 by a statement of the theory of evolution, direct- 

 ing attention to unwarranted inferences commonly 

 drawn bv clever writers unacquainted with the study 

 of nature. He described how the chantre in the character 

 of the struggle for existence, possibly in the Lower 

 Miocene period, which favoured an increase in the 

 size of the brain in the great mammals and the horse, 

 probablv became most important in the development 



NO. 1 86 1, VOL. 72] 



of man. The progress of man cut him off from the 

 general operation of the law of natural selection as 

 it had worked until he appeared, and he acquired 

 knowledge, reason, self-consciousness, and will, so 

 that " survival of the fittest," when applied to man, 

 came to have a meaning quite different from what it 

 had when applied to other creatures. Thus man can 

 control nature, and the "nature-searchers," the 

 founders of the Royal Society and their followers, 

 have placed boundless power in the hands of man- 

 kind, and enabled man to arrive at spiritual emanci- 

 pation and freedom of thought. But the leaders of 

 human activity at present still attach little or no im- 

 portance to the study of nature. They ignore the 

 penalties that rebellious man must pay if he fails to 

 continue his study and acquire greater and greater 

 control of nature. 



Prof. Lankester did not dwell upon the possible 

 material loss to our Empire which may result from 

 neglect of natural science ; he looks at the matter as 

 a citizen of the world, as a man who sees that within 

 some time, it may be only 100 years, it may be 500 

 years, man must solve many new problems if he is 

 to continue his progress and avert a return to nature's 

 terrible method of selecting the fittest. It seems to 

 us that this aspect of the question has never been 

 fully dealt with before. Throughout Huxley's later 

 writings the certainty of a return to nature's method 

 is always to be felt. Prof. Lankester has faith in 

 man's power to solve those problems that seem now to 

 be insoluble, and surely he is right. 



The dangerous delay now so evident is due to the 

 want of nature knowledge in the general population, 

 so that the responsible administrators of Government 

 are suffered to remain ignorant of their duties. Prof. 

 Lankester shows that it is peculiarly in the power of 

 such universities as O.xford and Cambridge, which 

 are greatlv free from Government control, to establish 

 a quite different state of things from that which now 

 obtains in England. He says : — " The world has seen 

 with admiration and astonishment the entire people of 

 Japan follow the example of its governing class in 

 the almost sudden adoption of the knowledge and 

 control of Nature as the purpose of national educa- 

 tion and the guide of State administration. It is 

 possible that in a less rapid and startling manner our 

 old Universities may, at no distant date, influence the 

 intellectual life of the more fortunate of our fellow 

 citizens, and consequently of the entire community." 

 Considering Oxford more particularly, and speaking 

 for others as well as himself, he sa^'s : — " The Uni- 

 versity of Oxford by its present action in regard to 

 the choice and direction of subjects of study is 

 exercising an injurious influence upon the education 

 of the country, and especially upon the education of 

 those who will hereafter occupy positions of influence, 

 and will largely determine both the action of the 

 State and the education and opinions of those who 

 will in turn succeed them." As to Greek and Latin 

 studies, he says : — " We have come to the conclusion 

 that this forni of education is a mistaken and in- 

 jurious one. We desire to make the chief subject of 

 education both in school and in college a knciwledge 

 of Nature as set forth in the sciences which are 

 SDoken of as physics, chemistry, geology and biology. 

 We think that all education should consist in the 

 first place of this kind of knowledge, on account of 

 its commanding importance both to the individual 

 and to the community. We think that every man of 

 even a moderate amount of education should have 

 acquired a sufficient knowledge of these subjects to 

 enable him at any rate to appreciate their value, and 

 to take an interest in their progress and application 

 to human life." He points out that it is only in the 



