238 



NATURE 



[July 6, 1S93 



but serving to promote the oxidation of the sulphurous oxide 

 at a temperature considerably below that at which sulphuric 

 oxide decomposes when heated. The action of surfaces gener- 

 ally may well be of this characler, and the converse influence 

 they so frequently exercise is probably an effect of the same 

 order. 



I have elsewhere raised the question whether there may not 

 be a difference between actions taking place under the influence 

 of low and of high electromotive forces — whether water, per se, 

 may not be an electrolyte towards high, although not towards 

 low, forces, in the case of high temperature changes, or those 

 brought about under the influence of the electric spark, for 

 example. More attentive consideration of the subject has led 

 me to think that this is not the case, and that we must treat 

 high temperature changes such as occur and are involved in 

 gaseous explosions in the same way as those occurring under 

 ordinary conditions and at low temperatures. From this point 

 of view, Mr. Baker's statement that ammonia and hydrogen 

 chloride do not combine is of extreme importance ; the forma- 

 tion of ammonium chloride from these two compounds appa- 

 rently involves no interchange, but a mere combination of two 

 substances each endowed with considerable "residual allfinily," 

 and there is no reason why a distinction should be drawn be- 

 tween such a case and that afibrded by, say, alonis of hydrogen 

 and oxygen, the difference being, it would seem, one of degree 

 only ; in fact, I am no longer inclined to believe that atoms are 

 capable of directly uniting. In all cases at least one function 

 of the (composite) electrolyte would appear to be that of pro- 

 viding the necessary " mechanism " whereby the degradation or 

 discharge of the energy is eff'ected. If this argument be sound, 

 its logical extension involves the conclusion that /«;v gases 

 should be dielectrics, i.e. that the passage of an electric dis- 

 charge through a gas like that of an explosive wave through, 

 say, a mixture of hydrogen and oxygen, can only take place if 

 an electrolyte be present. Hitherto but little attention has been 

 paid to the electric discharge in gases which have been highly 

 purified. The peculiar behaviour of Tesla tubes referred to by 

 Mr. Crookes in the discussion on Mr. Shenstone's paper on the 

 formation of ozone i>, perhaps, explicable from this point of 

 view — it may be that the atmosphere within the tube ^oes not 

 become conducting until sufficient moisture and "impurity" 

 have been projected from its sides. It is conceivable that a 

 similar explanation may hold good in the case of Prof. Schuster's 

 observation, that it is possible to urge a current of low electro- 

 motive force across a gas subjected to a high electromotive force 

 in itself insufficient to cause a discharge in the gas ; the.atomic 

 dissociation hypothesis put forward in explanation of the pheno- 

 menon does not appear to me to be sufficient. 



Finally, the question arises. Can no line be drawn ; are no 

 two pure substances capable of combining or interacting : — For 

 example, water and sulphuric anhydride? There is little to 

 guide us here, but it seems not unlikely that water has special 

 properties which enable it to act directly ; moreover — perhaps 

 because — in such cases composite electrolytes would result. 

 Ammonium chloride, so long as it remains solid, is clearly a 

 compound of a different order, and it may well be that com- 

 pounds of this type are in no case directly obtainable from their 

 constituents, because, under the conditions under which they 

 are formed, they cannot behave as electrolytes. 



Apparently, in all cases in which molecular aggregates are 

 formed — as in the case of solutions — we are dealing with dis- 

 sociable and dissociating systems, and it is not improbable that 

 we may ultimately find an explanation of the mechanism of such 

 changes in this fact. 



At present there is no information forthcoming whether simple 

 electrolytes, such as fused silver chloride, for example, will con- 

 dition chemical change in the way that water does — whether, 

 for instance, siWer chloride will condition the formation of 

 hydrogen chloride from chlorine and hydrogen, so that a gas 

 battery might be constructed of these three substaijces. 



Henry E. Armstrong. 



THE SUCCESSION OF TEETH IN MAMMALS. 



pROF. H. F. OSBORN, in the American Naturalist for 

 ■*• June, gives an account of recent researches upon the sue- 



sion of the teeth in mammals. He says : — 



"The recent studies of Kiikenthal, Rose, and Taeker in the 

 d iscovery of the complete double or milk dentition in the Mar- 



NO. 1236, VOL. 48] 



supials, and in the discussion of its relation to that of the rep- 

 tiles, also in (he ontogenesis of the crowns of the teeth among 

 the Cetaceans, Edentate?, Primates, and Ungulates are of the 

 greatest interest and importance. They involve a complete 

 revolution in our ideas as to the interpretation of the dentition 

 in the three orders first mentioned above." 



After giving an account of the work done by the European 

 observers. Prof. Osborn shows, by means of a table, the phylo- 

 genetic order as observed by Cope and Osborn, and the onto- 

 genetic order as observed by Rose and Taeker. His researches 

 indicate that the earliest forms of mammals were homodont, 

 and had two or more series of successional teeth. Then within 

 the mammalian stem the teeth were differentiated, and there 

 arose a great heterodont group with teeih at least of three kinds 

 — incisors, premolar^, and molars, all successional. From the 

 most anterior premolar arose the canine. Then came the division 

 between the Marsupials and the Placental', the former tending to 

 suppress the development of the second series of teeth, the 

 latter retaining the second series as far back as the firjt molar. 

 There is an obvious advantage in the line of succession bting 

 drawn at the first molar,' for upon the molars rested the neces- 

 sity of complex development, and such development was best 

 effected in permanent crowns. 



1. All the so-called "milk molars" plus the so-called 

 "true molars" constitute the /frj/ ic/-!(j. Beneath one or more 

 of the "true molars" in lower mammals are rudiments of a 

 second series. The second series consists therefore of these sub- 

 molar rudiments plus the successional or permanent premolais, 

 incisors and canines. 



2. In the stem Marsupials the entire first series persisted and 

 became mainly permanent (non-deciduous) ; the second series 

 became rudimentary and non-successional with the exception of 

 the fourth upper and lower premolars, and possibly one or two 

 other teeth which either replaced or were intercalated between 

 members of the first series. One or more premolars were sup- 

 pressed, and one more molar retained than typical in the Placen- 

 tals. Thus is explained the apparently atypical dental formula 

 of Marsupials. 



3. In the stem heterodont Placentals (excepting the Cetacea 

 and Edentata) the entire first series persisted, and all the in- 

 cisors, canines, and premolars remained deciduous. The suc- 

 cessional second series persisted as far back as the first molar. 



4. In the stem Cetacea the entire first series persisted, and 

 the second series became rudimentary and non successional. 

 The tooth form changed from a heterodont to a homodont tyj>e. 



5. In the stem Edentates, which also transformed from the 

 heterodont to the homodont type, the first series became rudi- 

 mentary, and the second series persisted in the succession even 

 behind the region of the first molar. 



Finally, there is evidence that a primitive succession in the 

 region of the molar teeth, lost in the Marsupials and in the 

 Placentals, was more or less fully retained in the Cetacea and 

 Edentates. 



UNIVERSITY AND EDUCATIONAL 

 INTELLIGENCE. 



The Governors of the Glasgow and West of Scotland 

 Technical College have appointed Mr. W. H. Watkinson, 

 lecturerer on engineering, Central Higher Grade School, 

 Sheffield, to the Chair of steam, steam engines, and other prime 

 movers, recently instituted in the college. By several impoitant 

 changes, the engineering department has been recently re- 

 organised, Prof. Jamieson devoting his attention entirely to 

 electrical engineering, Prof. Rowden to mechanics (theoretical 

 and applied), Prof. MacSay to machine drawing, and Prof. 

 Watkinson to the subjects stated above. With this addition 

 and rearrangement the college now possesses an engineering 

 slafr worthy of one of the greatest engineering centres in the 

 kingdom. Many additions are wanted, however, to bring the 

 labjratories and general equipment to a position of equably with 

 those even in many provincial towns. 



1 The law of molar evolution is Ihit complication is most rapid in teeth 

 which are longest in use. Thus the first mol.ar is the 'n"^' P'°f4;''" 

 tooth of the true molar series, and the last premolar is the ""' !^'/'°Sressive 



n that the thud milk pre- 



of the premolar scries. The apparent exceptio 



by 



plained 



olar is al« ays an advance type of the third permanent premolar is «P1»'°" 

 1 the fact that the milk nremolars are formed to assume the molar function 



