308 



KNOWLEDGE. 



August, 1913. 



THE TESTING OF DISINFECTANTS.— The usual 

 method of testing the value of disinfectants is by comparing 

 their germicidal powers under standard conditions. This 

 test, which is commonly known as the Rideal-Walker test, has 

 recently been subjected to severe criticism by Messrs. Kingzett 

 and Woodcock (Analyst, 1913, XXXVIII, 190), who cite the 

 results of experiments to show that the conclusions afforded 

 by the method are fallacious. For example, they point out 

 that such powerful chemical agents as nicotine, prussic acid, 

 and strychnine have little or no action upon the typhoid 

 bacillus, and that corrosive sublimate has a much lower value 

 than many coal-tar preparations when tested by the bacterio- 

 logical method. Again, copper sulphate, which is known to 

 possess germicidal powers, appears in the light of this test to 

 be nearly inert. In the opinion of these chemists the Rideal- 

 Walker test does not take into account variations in the chemical 

 conditions, and they consider that the only reliable method of 

 examining disinfectants as a class is to test them for the 

 particular purposes for which they are required. At the same 

 time they consider that there is no doubt that for coal-tar 

 disinfectants the Rideal-Walker test is the best that has yet 

 been devised. 



GEOGRAPHY. 



By A. Stevens, M.A., B.Sc. 



TOPOGRAPHY, EARTH - MOVEMENTS AND 

 ISOSTASY — In 1909 the United States Coast and Geodetic 

 Survey began publication of a series of bulletins* on Isostasy 

 and the " Anomalies of Gravity," which may be said to 

 develop along one line the work begun by Bouguer at 

 Chimborazo, and Maskelyne at Schiehallion. The possibilities 

 of the principle of isostasy in accounting, in part at least, for 

 the configuration of the land surface of the globe have been 

 for some time generally, if darkly, seen. But recently the 

 subject has been more closely studied, and interesting accurate 

 determinations have been made by Americans of the degree 

 to which isostasy obtains, and of the amount of correspondence 

 between anisostatic conditions and the irregularities of the 

 terrestrial surface. 



In these official publications isostasy was defined, a con- 

 venient and approximately correct upper limit of isostatic 

 equilibrium fixed relative to sea-level, and accurate determina- 

 tions in dynes of the actual intensity of gravity given for 

 upwards of a hundred and twenty stations in the United 

 States. For these stations theoretical values of the intensity 

 were computed by three methods : that of Bouguer, the free- 

 air method, and a new method due to Hayford, corrections 

 being applied for topography and compensation. The deficiency 

 or excess of the observed value over this computed value of 

 the intensity is called the " anomaly of gravity " for the station. 

 The United States have been mapped in areas of deficient, or 

 excessive gravity intensity, and the rate of variation shown by 

 contouring. 



As elsewhere, in the United States there is evidence of 

 differential vertical movement in the crust of the earth. In 

 particular the terracing of the Atlantic continental slope, the 

 submarine canyons and valleys indenting the Atlantic and 

 Pacific coasts, and the Post-Glacial deformation of the raised 

 beaches of the great lakes demonstrate extensive subsidence. 

 Deep valleys cut in the Tertiary deposits of Louisiana to 

 depths of between two and three thousand feet, which have 

 been filled with material containing shells representing living 

 species only, prove recent subsidence at the coast to the 



extent of two thousand three hundred feet, increasing inland, 

 it is estimated, to three thousand feet. 



Post-Glacial deformation has frequently been ascribed to the 

 tendency towards isostatic equilibrium asserting itself on the 

 disappearance of the ice. J. W. Spencert uses the anomalies 

 of gravity to show the inadequacy of the weight of the ice" 

 cap to counterbalance the earth-pressures involved. Gravity 

 anomalies are worked out by means of constants, due to 

 Bowie, to equivalent rock -thicknesses, and between the figures 

 so obtained and the amounts of subsidence and distortion 

 remarkable correspondence is observed. 



Spencer attributes the topographical "bulges" to differential 

 sinking conditioned by crustal rigidity, and invokes as the 

 prime cause of movement shrinkage of the earth beneath the 

 oceans and continents. Thereby continents are raised and 

 then gradually they sink towards isostatic conditions, pre- 

 serving on their submerged borders the marks of the 

 denudation they suffered in the elevated condition. This 

 explanation, of course, in itself presents little that is new, 

 and possibly may require revision in the light of work in other 

 fields of research. But it is important that a definite and 

 quantitative study of some topographic questions has been 

 found possible and initiated. Further work, of which we 

 have a promise, will be looked for with interest. 



GEOLOGY. 



By G. W. Tyrrell, A.R.C.Sc, F.G.S. 



AUSTRALITES. — The curious little bodies of volcanic 

 glass found scattered throughout Australia, and known as 

 australites, are the subject of a memoir by E. J. Dunn 

 (Bulletin No. 27, Geological Survey of Victoria). These 

 bodies are mostly button-shaped, with a central core and 

 depressed rim. The latter occurs only in the more perfect 

 forms owing to the accidents of weathering. These discoidal 

 forms are by far the most abundant ; rarer shapes are elon- 

 gated cylindrical and dumb-bell shaped ; others are quite 

 irregular. The material of the australites is acid volcanic 

 glass, identical in structure and composition with obsidian. 

 Thin sections show that the australites have good flow- 

 structures, and have been built up by material which flowed 

 in from above and around their peripheries. 



These bodies are regarded by Mr. Dunn as the blebs or 

 lower portions of glass bubbles. They are very similar in 

 form to the drop of water which collects at the base of a soap- 

 bubble. They are conjectured to have been formed in 

 volcanoes, and their distribution over Australia is accounted 

 for by their dispersal through the agency of air-currents. The 

 bubbles would be blown up to a height of five or six miles 

 above the volcano, and before their destruction might have 

 been carried hundreds of miles from their point of origin by 

 the air-currents of the upper atmosphere. On the bursting 

 of the bubbles the pendant blebs would fall to the ground, 

 and would become embedded in whatever formation was then 

 in process of deposition. 



Australites occur in deposits representing a period corre- 

 sponding with that covered by the last great episode of volcanic 

 activity in Australia. It is uncertain, however, how far they 

 extend back into the Tertiary era. They were formerly 

 regarded as of meteoritic origin ; but this cannot be reconciled 

 with their chemical composition, which is utterly unlike that 

 of any meteoritic body, with their symmetrical shapes, and 

 local distribution. 



J. F. Hayford, " The Figure of the Earth and Isostasy from Measurements in the United States' 



" (U.S.C. # G. Survey, 1909); J. F. Hayford and W. Bowie, "Effect of Topography and 



in 1909 of the Figure of the Earth and Isostasy 



Isostatic Compensation upon the Intensity of Gravity " ; W. Bowie 



the Intensity of Gravity, Second Paper ' 



A Supplementary Investigation 



1 Effect of Topography and Isostatic Compensation upon 

 (U.S.C. & G. Survey. 1912). 



1 J. W. Spencer, " Relationship between Terrestrial Gravity and Observed Earth Movements in Eastern America," Amcr. four. 



Science, June, 1913. 



