April, 1904.1 



Tlu 



KNOWLEDGl*: \- SCIENTIFIC NEWS. 



57 



well. 



i.ij lui- Litciiii may. as wen. serve as a 

 telautographical recciviiif;-apparatu:s. i.e. an apparatus for re- 

 producing handwriting, drawinsr. &c.. at groat distances. In 

 this case, only sonic ver\' slight alterations will liave to be 

 made, and .i BakewcllCaselli transmitter used. The 

 speed attained is relatively very high. It has been loinid 

 possible to reproduce from tueiitv to fortv words in the 



Orig:inal Photograph. 



original handwriting iu the course of \.hu-e minutes, and in th< 

 case of shorthand much higher speeds may be arrived .it. 

 The transmis.sion of photographs, of course, is slower, princi- 

 pally on account of a certain inertia of the selenium. Tin- 

 progress lately made in connection with the construction of 

 seleniimi cells', however, makes much higher speeds very prob- 

 able. The time at present required for telepholographing a 

 portrait is about half-an-hour. 



Transmitled Photograpli. 



Figs. 2 and 3 show the telegraphic reproduction of a photo- 

 graph and a telautographic specimen respectively. The in- 

 ventor wishes us to state that part of the imperfections of tlie 

 photo, especially the stripes, is due to the e.xperiments having 

 been made in the Physical Laborator\- of the Munich Univer- 

 sity, where the pressure of the battery and, accordingly, the 

 intensity of the source of light, would undergo frequent fluctua- 

 tions. 



Modern Views of 

 CKemistry. 



By II. J. II. l-E.MON, l-'.K.S. 



In our last coiiunimiiation \\c indicated \er\ Ijrieth', in 

 outline, the nature ol the ionic-dissuciatiiui Ii\ polhesis, 

 and mentioned Slime of the expeiiniental fads upon wbicli 

 it is basi'd ; we propose now to ^i\e a few illustr.il ions oi 

 the manner in, which the lupothesis li;is been applied to 

 the explanatioir or inlerpret.ilioii ol sonic well known 

 chemical and physical laiis. 



\\ hat is an ;u'id r' l'!\ei\one who is al all ai(|uamled 

 with the cleinenlar\ huts ol cheniislrx has a lairh clear 

 conception in his own mind what the term implies, but 

 attempts to frame an exact definition .are not always 

 satisfactory. If an acid is " an\' liydroi^eii compound 

 which can exclKuii.j(; its liy(ir<)<ien, wholly or [lartly, for a 

 metal when the latter is presentetl to it In llie form of a 

 hydroxide," we must include as acids substances suih as 

 zinc hydroxide and aluminium hydroxide, the distinction 

 between acid and base heiny relative rather than .abso- 

 lute. It was at one lime proposed lO' restrict the term 

 " true acid " to a. c,om|)oiui(l which can behave in the 

 abo\e manner even in presence of much water, and such 

 a restriction woidd, it is true, exclude substances like 

 zinc hydroxide, but it would al.soi exclude some com- 

 [loimds like silicic acid which are looked upon as acids. 

 Other definitions, such .is " a salt of hydrogen," " a 

 compound which can evolve water by its action, 011 

 caustic potash," or " a compound of hydrofjen with an 

 elect ro-ne£^ative element or group, ' ("an generally b(^ 

 foimd f;uilt with, and there is often a tendency to define 

 the terms " acid," '' salt," " base " in a circle. 



The ionic-dissociation hypothesis now comes to the 

 rescue with an elegant and .simple definition. ,\n acid, 

 it says, is :\ ccuiipoimd whose aqueous solution contains 

 free hydrogen ions. \\'hat we call acidity or acid-pro- 

 perty in a solution is due to these ions, and is more prf>- 

 noiinced as their concentration is greater, /.(■., the more 

 there are in a gi\en \-olunie. A base, on the other li.uid, 

 is a compound wlio.se aqueous solution conlains Irec 

 Indroxvl (() II) ions, and when an acid neutralises a 

 base the only change which takes place (provided the 

 solution is dilute and the acid and base are " strong ") is 

 the union of the free hxdroxyl and hydrogen ions to 

 form water. It will beobser\ed that, according to this 

 conception of the m.itter, neither the metal or acid 

 radicle takes any part in the change ; they remain as 

 free ions throughout — 



H + K + M + O H =M -f- R -f- 11 H 



(where K is the acid radicle and M the metal). 



It must not be forgotten that the older definitioiis 

 alluded to above are practical ones, whei^eas thi.s ionic 

 definition depends entirely upon hypothesis ; the latter, 

 however, affords a remarkably simple explanation of 

 many well-known facts. When, for example, equivalent 

 weights of strong acids (say, hydrochloric or nitric) 

 neutr.-ilise strong bases (say, caustic potash or soda), the 

 quantitv of heat evolved is always the same. This fact 

 is easilv understood on the above supposition, since in 

 each case the only change in the arrangement is the 

 union of hvdroxyl with hydrogen. 



If the ;icid or base, or both, are not " strong,'' the heat 

 change on neutralisation w ill be different from that in the 

 previous case. Thif. is explained by saying that the 

 weaker acids and bases arc not entirely in a state of 



