T h e properties o f the clouds p ro d u cin g Ly a absorption an d G iv absorption in the spectra o f quasars are review ed. T h e G iv -p ro d u cin g clouds m ay be classified into th ree groups representing ejection, clouds m oving in a cluster co n tain in g the q u asar itself, an d interv en in g m aterial a t cosmological distances. T h e incidence o f th e la tte r type im ply effective rad ii for lum inous galaxies o f o rd er 100 kpc, an d a present-epoch m ean free p a th o f ca. 10

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... G pc. No decisive evidence y et exists a b o u t w h ether m ost o f th e L y a p ro d u cin g clouds arise in tru ly p rim o rd ial hydrogen clouds as opposed to m etal-enriched m a terial associated w ith galaxies. In th e la tte r in terp retatio n , the line density is such th a t th e m ean free paths are 100 times sm aller an d th e effective galaxy ra d ii 10 tim es larg er th a n for G iv. A t a b o u t the tim e th a t the first quasars w ith large red shifts w ere discovered, it was realized by a n u m b e r o f people, n o tab ly G u n n & Peterson (1965) th a t the presence or absence o f absorption in q u asar spectra co n tain ed v alu ab le inform ation concerning the interg alactic m edium . U n fortunately, th e im p o rtan ce o f the test was quickly overshadow ed by th e controversy over w h eth er or n o t the q u asar emission line red shifts w ere actually cosmological. This controversy now seems to have nearly ru n its course, especially in the light o f the recent com prehensive w ork by Stockton (1978) on th e association o f quasars w ith sm all red shifts w ith sm all groups o f galaxies. I n the rem ain d er o f this co n trib u tio n we shall therefore assume the cosmological n a tu re o f th e red shifts an d ask w hat, if anything, absorption by gas clouds betw een ourselves an d the quasars can tell us a b o u t uncondensed gas in th e U niverse. U n c o n d e n se d n e u t r a l h y d r o g e n T h e sim plest form o f th e test for intergalactic n eu tral hydrogen sim ply assumes th a t th e gas is distributed uniform ly along the line o f sight betw een us an d th e quasar, an d has no appreciable ran d o m velocity o f its own. Photons em itted to th e blue o f the Ly a line in the con tin u u m o f a qu asar w ould th en be scattered o u t o f the beam , form ing a sm ooth absorption tro u g h from Ly a tow ards the blue. T h e effective optical d ep th a t an epoch corresponding to some red shift z is th e n : j* +zr*. ( 1) In practice, this test is com plicated by the fact th a t the co n tinuum o f m ost individual quasars w ith large red shifts are cu t up by num erous absorption lines a t shorter w avelengths th a n Ly a. f We use H0 -50 km s'*"1 Mpc~1 and a % -% cosmolo 26-2