NATURE 



[December 29, 1904 



SaXti and their Reactions. By Dr. L. Dobbie and H. 

 Marshall. Pp. iq8. (Edinburgh : James Thin, 

 iqo4.) Price 3s. 6d. net. 



This bcx)k is intended to serve as an introduction to 

 the study of practical chemistry, and has for its basis 

 a series of notes intended for use in the Edinburgh 

 classes. In an interesting preface Prof. Crum Brown 

 states his belief in the possibility of devising a course 

 that would be " something better than a mechanical 

 training to enable students to pass a mechanical ex- 

 amination consisting in the detection of simple salts 

 in solution." Notwithstanding this assurance, one 

 finds that about half the book consists of descriptions 

 of the ordinary tests and schemes of analysis common 

 to most books treating of elementary practical 

 chemistry. 



The first part of the work consists of a short and 

 very clear account of the general physical properties 

 of salts and salt solutions. An outline is given of the 

 ionisation hypothesis and of its applications, some of 

 which are practically illustrated at a later stage. 

 After a short account of the nature and use of in- 

 dicators, a chapter is devoted to alkalimetry and 

 acidimetry. The experimental part of the book, ex- 

 cluding the sections on qualitative analysis, is only 

 represented by about twenty-five pages, and although 

 the selection of experiments has evidently been care- 

 fully made, it seems a pity that the practical illustra- 

 tion of a really excellent theoretical introduction should 

 be so meagre. 



The remainder of the book is taken up with a de- 

 scription of the reactions of metallic and salt radicals, 

 and with schemes for analysis. In several small par- 

 ticulars a departure from the conventional methods 

 has been made with distinct advantage. Dry-way 

 reactions, which so few chemists appear to appreciate, 

 are relegated to an appendix, which also contains the 

 inevitable and perfectly useless description of the re- 

 actions of the so-called rare elements. Teachers who 

 have the management of large practical classes should 

 find the volume of value. 



LETTERS TO THE EDITOR. 



[The Editor does not hold himself responsible for opinions 

 expressed by his correspondents. Neither can he undertake 

 to return, or to correspond -vith the writers of, rejected 

 manuscripts intended for this or any other part of X.ature. 

 No notice is taken of anonymous communications.] 



Radiation Pressure. 



On p. 515 of your issue of September 22 I stated that 

 there is a retarding force on the earth as it moves along its 

 orbit amounting in all to about 20 kgm. The calculation 

 was made on the supposition that the earth is a full radiator 

 of uniform temperature. I have found on revising the 

 calculation that there was an error in the arithmetic, and 

 that the force is considerably greater, though still too small 

 to have an effect worth considering. The following is a 

 simple method of obtaining its value. It assumes that the 

 earth may be treated as a black sphere exposed to sun- 

 light, radiating as much as it receives, and with all its 

 surface at one temperature. 



If the stream of solar energy falling normally on 

 I sq. cm. is S per second, a black sphere, radius a. 

 receives Tra'S per second. If it radiates R per second per 

 sq. cm. its total radiation is ^-ra-R, and the assumption of 

 equal receipt and expenditure gives R = S/4. The total 

 repulsive force exerted by the sun's radiation is Sna'/V, 

 where U is the velocity of light. The total retarding force 

 due to velocity 11 in the orbit is 4/3 Ru/U^.to^ This is the 

 Doppler effect due to crowding of energy in front and open- 



NO. 1835, VOL. 71] 



ing out behind (Phil. Trans., A, ccii. p. 546, corrected by 

 final note). Hence we have 



Retarding force. 

 Solar repulsion 



3U" 



.\t the earth's distance «/U is about 10-', so that the re- 

 tarding force is about 1/30,000 of the solar repulsion. 



If we take S/U as s-Sxio-" dyne/sq. cm. (Phil. Trans., 

 loc. cit., p. 539), and the radius of the earth as 

 637x10' cm., the total solar repulsion is about 

 75x10' kgm., say 75,000 tons, and the retarding force is 

 about 2500 kgm. 



But another effect comes in which will more than counter- 

 balance this. The hemisphere of the earth which is 

 advancing in the orbit is on the whole colder than that 

 which is retreating, owing to the lag in the warming of 

 the surface exposed to the sun. I find that if one hemisphere 

 is at 301° A. and the other at 300° A., the greater radiation 

 from the warmer side gives a net push directed from that 

 side to the colder of about 165,000 kgm. Of course this 

 hemispherical distribution of temperature is only a rough 

 approximation to the real condition, and even if the force be 

 as large as 165,000 kgm. only a component of it acts along 

 the orbit tending to accelerate the motion. Still, that com- 

 ponent must almost certainly be much greater than the re- 

 tarding force due to the Doppler effect, and on the whole, 

 therefore, there is probably a small acceleration in the orbit. 

 A force of 2500 kgm. would destroy about 4 10" of the 

 earth's momentum in one year. Even if the accelerating 

 force were twenty-five times as great as this it would only 

 generate i/io" of the present momentum in one year. 

 This illustrates the insignificance of radiation pressure on 

 the larger bodies in the solar system. 



I take this opportunity of correcting another error in the 

 address in N.\ture of September 22, which has been pointed 

 out to me by Mr. C. T. Whitmell. It arose from some very 

 faulty arithmetic on p. 541 of the paper in the Philosophical 

 Transactions already referred to. .'Apparently in the formula 

 giving the radius of each of two equal spheres the mutual 

 radiation-repulsion of which balances their gravitative 

 attraction, a square root of 10 was omitted, and the value 

 of that radius should be a = o 699-/io''p. .\ wrong value 

 was also assigned to the density of the sun. Mr. Whitmell 

 has very kindly re-calculated the results depending on this 

 formula, and I have worked them out independently. We 

 now find that two equal spheres will have equal radiation- 

 repulsion, and gravitative attraction with radii as given 

 below : — 



6200 

 300 

 300 

 300 



55 



1930 

 61 



0-5645 

 ■13 



The last was given previously as 34 cm. 



The effect of radiation pressure on terrestrial dust is 

 worthy of consideration, for it may be quite appreciable 

 when the particles are small and are among surroundings 

 at different temperatures. For simplicity of calculation, let 

 us suppose a very small dust particle, of density p, to be 

 cylindrical with radius a and length a. and let its flat ends 

 be black and let its curved surface be perfectly reflecting. 

 Let it be situated between two indefinitely extended parallel 

 vertical walls, one at a temperature 0,° .\.. the other at a 

 lower temperature 9,° A., and let its ends be parallel to the 

 walls. The two faces of the dust particle will, if it is small 

 enough, be at very nearly the same temperature, so that 

 we may leave out of account the pressures due to the emitted 

 radiation and consider only those due to that received from 

 the walls. If ff is the radiation constant 5-32 X 10-^, and if 

 U is the velocity of light, the difference of pressure on the 

 two sides will be 2it(9/-9/)/3U, and the acceleration due 

 to this on area va' and mass prra' is 2(T(9,'-9/)/3Upa. 

 When p = i, a=io-', 9, =400° A., f2 = 30O° A., this acceler- 

 ation is 002 cm. /sec.'. 



