portance in Silicosis P revention,” published in Industrial and Engineering Chemistry, Anal. Edition, 11, 583 (1939).
1 J. J. Denny, W. D. Robson and D. A. Irwin, Canadian Medical Association Journal, 37, 1-11 (1937); 40, 213-228 (1939).
152
A P P L I C A T I O N OF E L E C T R O N D I F F R A C T I O N 153 sten ribbon, th e d istan c es an d th e q u a n titie s of gold an d silica hav in g been a d ju ste d so th a t th e resu ltin g com posite film consisted of a layer of silica of thick n ess 2 X 10~6 cm lying upon a lay er of gold of thick n ess 30 X 10-6 cm . T h is com posite film was large enough to su p p ly a g rea t m a n y sam ples of silica w hich could be used in a large n u m b e r of experim ents. E a ch sam ple w as p rep ared , as an d w hen req u ired , by strip p in g from th e glass slide a sm all piece of th e com posite film, d is
solving th e gold in a n itric-h y d ro ch lo ric acid m ix tu re, a n d th en w ashing th e rem ain in g tin y silica film in several changes of d istilled w ater.
F ilm s p rep ared in th is m a n n e r w ere floated upon distilled w a te r co n ta in in g alu m in u m pow der, for v ario u s lengths of tim e an d a t tw o
differ-A B
Fig. 1—Electron diffraction patterns from a relatively thick layer of oriented alum inum alpha-m onohvdrate crystals formed upon a silica film as a result of ex
posure of the film to metallic alum inum and w ater a t 38° C. A — Electron beam normal to film surface. B— Beam inclined 45° to film surface.
e n t te m p e ra tu re s. In som e e x p erim en ts th e p H of th e w a te r w as a d ju ste d b y th e a d d itio n of HC1 or v ario u s salts.
F ilm s tre a te d a t 38° C. (ap p ro x im ately bod}' te m p e ra tu re ), a n d a t m edium an d high p H values 2 (6 to 9), gave sh arp electro n diffraction p a tte rn s w hich w ere identified w ith o rien ted cry stals of th a t h y d ra te d oxide of alu m in a know n as alu m in u m alp h a -m o n o h y d ra te (B oehm ite).
T y p ical p a tte rn s a re rep ro d u ced in Fig. 1. A t a low p H v alu e (pH 4) m o n o h y d ra te c ry sta ls were n o t discovered even a fte r long reactio n tim es. A lth o u g h th e c ry sta l stru c tu re of alu m in u m alp h a-m o n o - h y d ra te is n o t know n it w as possible to m ak e th e id en tificatio n by
2 T he term pH is defined as the logarithm of the reciprocal of hydrogen ion con
centration, hydrogen ion concentration being expressed for purposes of this definition in term s of gram s of hydrogen ions in a liter (or more strictly 1000 grams) of solution.
In a neutral solution pH = 7; in acid pH < 7 and in alkali pH > 7.
154 B E L L S Y S T E M T E C H N I C A L J O U R N A L
co m parison of th e electron p a tte r n s w ith X -ra y a n d electro n p a tte rn s o b ta in e d from th e b u lk m a te ria l (Fig. 2).
E le ctro n d iffractio n p a tte r n s from a lp h a -m o n o h y d ra te form ed on silica surfaces w ere found to v a r y m a rk e d ly w ith p H of th e alum i- n u m -w a te r so lu tio n a n d w ith th e rea ctio n tim e. F ro m th ese p a tte rn s th e follow ing conclusions w ere d ra w n . M o n o h y d ra te c ry sta ls form ed a fte r sh o rt reactio n tim es (4 h o u rs to 20 ho u rs) w ere sh a rp ly o rien ted w ith a p a rtic u la r c ry sta l p lane parallel to th e silica su rface; th e in d i
v id u al c ry sta ls w ere on th e av era g e fairly large (from 5 to 10 X 10~7 cm ) in d irectio n s parallel to th e surface, a n d th in (2 X 10-7 cm or less) n o rm al to th e surface. As th e reactio n tim e w as increased, th e c ry s
ta ls becam e, on th e av erag e, th ick er norm al to th e surface (b u t seldom
Fig. 2— Electron diffraction p attern obtained by the reflection m ethod from finely pulverized alum inum alpha-m onohydrate (AUOs-HiO).
as th ic k as 5 X 10“ 7 cm ), a n d a t th e sam e tim e o th e r c ry sta ls of m o n o h y d ra te w ere form ed w hich were less n e a rly p e rfectly o rie n te d a lth o u g h still show ing th e sam e stro n g preference. F o r long re a c tio n tim es lay ers o f co m p letely u n o rie n te d a lp h a -m o n o h y d ra te c ry sta ls w ere som etim es produced.
In th e presence of tra c e s of organic acids o rie n te d so ap c ry sta ls w ere form ed as a re su lt of th e reactio n of alu m in u m a n d w a te r. T hese cry sta ls w ere p ro d u ced a t all p H values. T h e y a p p e a re d as scum upon th e w a te r surface, a n d w ere n o t re ad ily ad so rb ed u pon silica.
T h is fa c t proves t h a t th e a ctio n of a lu m in u m in p re v e n tin g d ev elo p m e n t of silicosis c a n n o t be a ttr ib u te d to an alu m in u m soap. F ig u re 3 ex h ib its a ty p ic a l d iffraction p a tte r n from o rie n te d cry sta ls of an a lu m in u m soap.
A P P L I C A T I O N OF E L E C T R O N D I F F R A C T I O N 155
Fig. 3—Electron diffraction pattern produced by a layer of oriented crystals of an alum inum soap, which had been formed as scum upon a w ater surface as a result of the reaction of powdered alum inum , w ater and traces of organic acid present as an im purity.
O ur experim ents prove t h a t alu m in u m h y d ra te is p re c ip ita te d fairly rap id ly upon silica a t pH values lying w ithin a range in w hich lie also th e p H values of b ody fluids of m en a n d of anim als. Since in these experim ents alu m in u m h y d ra te is n o t form ed upon silica a t p H 4, i t seems highly p robable th a t alu m in u m w ould n o t afford p ro te c tio n from silicosis to a h y p o th e tic a l anim al w ith b ody fluids of p H 4.