OcTOHEn 2, 1.--98. 



KNOWLEDGE 



191 



atmosphere would double. No doubt the photosphere 

 would act as a screen, preventing radiation from the solar 

 nucleus, and the temperatures within the photosphere 

 would probably be much hif,'her than in the coronal region. 



The volume of a gas varies as ^--^ where r is the 

 absolute temperature in degrees Centigrade, and if the 

 solar atmosphere extends as high as the level of the photo- 

 sphere, the average temperature of the material within 

 the photosphere will need to be enormously great, for the 

 average density of the sun is only about a quarter as great 

 as that of the earth, viz., 1-44-1 times the density of water. 



Some vague idea as to such temperatures may be 

 gathered from the consideration that with a gas obeying 

 Boyle's law, and with gravity remaming uniform as at the 

 level of the photosphere, an atmosphere of hydrogen (at a 

 temperature of two million seven hundred and thirty 

 thousand degrees Centigrade, measured from the zero of 

 temperature —273° Cent.) would increase in density a 

 million million million times in descending from the 

 photosphere to a depth of 405,000 miles ; for other gases, 

 or a mixture of vapours, the rate of increase of density 

 would be still more rapid. But the assumption that gases 

 above their critical temperature when greatly compressed 

 continue to obey Boyle's law is probably not permissible, 

 for at low temperatures Boyle's law only approximates to 

 the truth when the free paths of gaseous molecules are 

 long compared with the molecular diameter. 



It is not, however, necessary to assume that the incan- 

 descent clouds of the photosphere are Heating in an 

 atmosphere, as clouds of water vapour float in our 

 atmosphere. In the case of terrestrial clouds, each little 

 globule of water is no doubt continually falling, but it falls 

 very slowly because of its small weight and the relatively 

 large surface which is exposed to the resistance of the 

 atmosphere. The weight of a small particle decreases as 

 the cube of its diameter, while the surface which it exposes 

 to a resisting atmosphere only decreases as the square of 

 its diameter ; consequently, the smaller the particle, the 

 slower will bo its rate of falling through a resisting at- 

 mosphere. 



A minute particle in the photosphere would be retarded 

 in its fall under the action of solar gravity by the backward 

 kicks of molecules evaporated from its under surface 

 exposed to the radiation from the heated centre. We 

 should expect to find such falling particles checked in their 

 downward course most rapidly just before they were finally 

 evaporated, for two reasons ; (1) because the weight of the 

 falling particle would have already been greatly sweated 

 away, and the total weight of the particle would bear 

 a smaller proportion to the weight of the molecules eva- 

 porated towards the heated centre in a unit of time, and 

 (2) because we should expect the force and number of the 

 backward kicks to increase very rapidly when the particle 

 enters a region where the heat is such that it will rapidly 

 be completely evaporated." 



We should therefore expect small particles, falling 

 towards a very hot body, to accumulate at a level where 

 their falling motion was thus rapidly checked, and if the 

 particles consist of matter which cannot be driven into 

 vapour until it has attained a white heat, we should expect 

 a layer of such particles to shine like a brilliantly incan- 



* The meau velocity of hydrogen molecules at a temperatiu'e of 

 2184° Cent, is .3"18 miles per second, and the mean velocit}- of hydi'o- 

 gen molecules evaporated from a surface heated to 2184° Cent, must 

 be much gi-eater than tlic mean velocity of molecules of hydrogen gas 

 at a temperature of 2184° Cent., for the swiftest molecules are the 

 first to separate themselves from the attracting molecules of a 

 heated sui-face. Hence evaporation, hy lowering the average velocity 

 of the molecules left behind, cools an evaporating surface. 



descent cloud. It is therefore conceivable that the 

 photosphere might lie far above the summit of the solar 

 atmosphere. But the great inclination to the radial of 

 many of the prominence structures thrown upwards into the 

 region of the chromosphere and corona seems to point to the 

 conclusion that the region from which the explosions take 

 place cannot be situated at a great depth below the photo- 

 sphere, and one can hardly conceive of explosions taking 

 place which result in driving matter outwards unless there is 

 some fulcrum to drive from, as there would be if explosions 

 took place within a solar atmosphere in which the resist- 

 ance to motion is greater in a downward than in an 

 upward direction. 



The great tenuity of the gaseous matter outside the 

 photosphere is, it seems to me, evidenced by the narrow 

 character and want of absolute blackness of the dark lines 

 in the Fraunhofer spectrum, as well as by the narrowness 

 of most of the bright lines, of the incandescent gases of the 

 chromosphere. These come from a lower region, probably 

 of greater pressure and temperature, and in the case of 

 great outrushes the chromosphere lines are sometimes 

 broadened and reversed in a complicated manner, im^Dlying 

 pressure and overlapping streams of gas at different 

 temperatures ; but as a general rule the lines of the 

 chromosphere spectrum are narrow, indicating long free 

 paths and uniform vibration of the radiating gaseous mole- 

 cules, such as only can be conceived of in a gas of extreme 

 tenuity. 



The dark lines of the Fraunhofer spectrum are, as a 



general rule, far from being absolutely black, proving that 



only a portion of the light of the wave-length radiated 



by the photosphere has been lost in its passage through 



the absorbing region ; whereas the old experiment of 



Kirchhoff with the flames containing sodium and lithium 



vapour, proves that a few inches, or decimal parts of an 



i inch, of vapour in which the molecules are comparatively 



' close together, is sufficient to give an amount of absorption 



which materially darkens the absorption lines produced in 



the whole thickness of the solar envelope. With absorbing 



molecules very widely and sparsely scattered, only a small 



portion of the wave front corresponding to a ray of light 



; would be interfered with in its passage through the 



absorbing region. 



S(wn« Notts. 



The microscopic hairs on the leaf of the hop would 

 appear to partake in a small degree of the properties of 

 those of the closely-related stinging nettle. The peculiar 

 form of eye trouble spoken of as hop-picker's ophthalmia, 

 seems at any rate to be ascribable to these hairs, which 

 adhere to the hands and subsequently get rubbed into the 

 eyes. , , , 



The fastest mechanism, artificial or natural, made to 

 penetrate water for any considerable distance, is according 

 to Mr. -Jeremiah Head, Thornycroft's torpedo boat, 

 " Arietc," which on trial made 30'16 miles per hour. 



M. Lionel Deck', in an account of the marriage customs 

 of the Matabeles, mentions one feature which is, we behave, 

 quite singular. The husband does not buy his wife, and 

 although there is a suggestion of a memory of marriage by 

 capture in the details of courting and in the avoidance of 

 the parents-in-law, she would appear to remain her father's 

 property. When children are born, the father has to buy 

 them of his father-in-law, or, failing this, they revert to 

 the mother's family. 



