diffraction of S-riiys I J ~ gtiscs rtttcl vnpotirs :itid the diffrnctioii pliciioiiienii which :ire obscrrcci wlicti I)c:iiiis of electrons iiiipiiigc oti ni:itter. In t,lic study of giiscs ;ind vnpo~irs by S-r;tys wc Iinw n ttic:iiis of titcnsiiring intcrntoiiiic (1ist:iiices i n iiiolccitles which :ire rcliiti\4y fiir reniowtl froiii the effects of tlicir ticighbours, ;is opposed to those itt :L solid nggrcgiitc. Nlectron diflrnct.ioii is also used in iiic:isiircniciit~s 011 gnscous niolcculcs, but has: in ntltlit.ioii, ii wide ficld of npplicntion in the ex;imiiint.ion of solids, prticnlnrly in relittion to tlic special problcms wliich iire presentad by surfaces. It might,, inilcetlt be claimed tlint, tliesc two cspcriuicntiil methods iirc uscfiil in-solving just tliosc prol)lcins in which the cliissicnl iiiethods of S-riiy spectroscopy fail, itiid many iiiiportniit ndviinccs hnrc nlrcndy 1)ceii niiitlc with their aid.
DIFFRACTION OF X-RAYS BY GASES AND VAPOURS
?'lie scutteriiig of X-riiys by trutttcr tlepcttds rtltiiiiiitcly on sriittcriiig liy t h electrons of the constituent iitonis. Each of bhc ntoiiis in itiiy one lnolccule must tlicrcfore act ;is ii centre of scnttcritig, a t i d tlie distiiriccs bctrwcii the sciittering cetit.rcs iti citcli Itlolccdc will h, to ii first ii pproxitii;ttion, tlic siiiiic its tlic ortlinitry intcriitoniic. tlistiitices. Sit jipose ~iow tlmt a 1)etini of S-rtiys enters ii spitre coiitiiinitig gas 1iiolcc:iilcs of soiiic pnrticulitr sort. l'liesr niolcculcs cr~cli coiitititi two or iiiorc centres riiiitlom iirrniigciiietit, with respect, to the iiiaitlctit S-rriy (IItOlllS) r i t ~J R ~J I C Of Sciittc~jll,q .,y-l'ilJ'S, l l l l ~~ tllCy hn.c il