[king] molecules in INTERSTELLAR SPACE 101 



For the purposes of the present discussion we assume hydrogen 

 to be the constituent of interstellar space (until we know more about 

 the physical properties of "coronium," "nebulium," or other primor- 

 dial gases which might possibly occupy these regions). Taking 

 Mo-l=0-000140, Wo = 2•78XlOl^ A = 4-46X10-» cm., we easily 

 derive for the coefficient of attenuation in hydrogen at standard 

 temperature and pressure the value iro = 5 -89X10-^ cm.-^. For this 

 wave-length in interstellar space we have X = 2- 72X10-^2 cm.-S 

 so that w/«o = -K/-^o = 4•62XlO-^^ giving finally for the molecular 

 density in interstellar space the estimate w = 1-28X105 Hydrogen 

 molecules per cm.^. ^'^ 



Associated with the problem of attenuation by scattering is that 

 of calculating the amount of star-light scattered by the molecules 

 of interstellar gas.^^ In this way might be explained the extremely 

 faint luminosity which several observers believe to exist over the 

 background of the sky. This scattered light might also account for 

 discrepancies wiiich have been found to exist between calculated and 

 observed distributions of total starlight from different regions of the 

 night sky. ^2 The estimation of the amount of solar radiation scattered 

 to the earth by a distribution of interstellar gas constitutes a definite 

 problem the complete statement of which (including the effect of 

 self-illumination) is expressed as a particular case by a general integral 

 equation already given by the writer. ^^ The theoretical discussion 

 applicable to the problem under discussion the writer hopes to under- 

 take elsewhere: from the observational point of view it would seem 

 that the difficult but perhaps not impossible task of estimating the 

 luminosity of the sky in regions void of stars affords the only hope 

 of bringing additional direct evidence to bear on some of the questions 

 raised below. 



In a gas of the extreme degree of tenuity which we have just 

 estimated, molecular collisions will be extremely infrequent; an esti- 

 mate of the free path according to the usual ideas of the kinetic 

 theory is impossible without a knowledge of the average molecular 

 velocity or temperature of the gas. As has already been pointed out 

 by the writer^* it is difficult to see how molecular velocities can be 

 directly affected by radiation travelling through a gaseous medium. 

 It is probable that gravitation and radiation-pressure are the control- 

 ling forces in ultimately determining molecular velocities by an 



" Note a discussion on this point by H. C. Plummer in a paper by Turner, H. H. , 

 oc. cit. (footnote 1). 



^Ahhot,C.G., Astronomical, J. 27, (1911), p. 20-: "Annals of the Smithsonian 

 Astrophysical Observatory," Vol. III. (1913), pp. 203-210. 



13 King, L. v., footnote (6), p. 379, equation (14). 



"King, L. v., footnote (7). 



Sec. III. 1915—7 



