688 report — 1884. 



A table was exhibited which showed the comparative characters, as to percent- 

 age of nitrogen and carbon, of exhausted arable soils, of newly laid-down pasture, 

 and of old pasture soils, at Rotharnsted ; also of some other old arable soils hi 

 Great Britain ; of some Illinois and Manitoba prairie soils ; and lastly, of some very 

 rich Russian soils. From these results there could be no doubt that a characteristic 

 of a rich virgin soil, or of a permanent pasture surface soil, was a relatively high 

 percentage of nitrogen and carbon. On the other hand, soils which have long been 

 under arable culture are much poorer in these respects ; whilst arable soils under 

 conditions of known agricultural exhaustion show a very low percentage of nitrogen 

 and carbon, and a low relation of carbon to nitrogen. 



In conclusion, the authors said it had been maintained by some that a soil was 

 a laboratory and not a mine ; but not only the facts adduced by them in this and 

 former papers, but the history of agriculture throughout the world, so far as it was 

 known, clearlv showed that a fertile soil was one which had accumulated within it 

 the residue of ages of previous vegetation ; and that it became infertile as thie 

 residue was exhausted. 



5. On the Velocity of Explosions in Gases. By H. B. Dixon, M.A. 



MM. Berthelot and Vieille have found that in hydrogen and oxygen, ethane 

 and oxv^en, and many other mixtures of gases, the ' explosive wave ' is propa- 

 gated with a velocity closely approximating to the mean velocity of translation 

 of the gaseous products of combustion, calculated en the assumption that all the 

 heat of the reaction is retained for the moment in the products formed. Thus 

 the mean of a number of determinations with electrolytic gas gave a velocity of 

 2 810 metres per second ; the calculated mean velocity of the steam molecule 

 formed being 2,831 metres per second. But with carbonic oxide, exploded either 

 with oxvgen or nitrous oxide, the velocity of explosion was much less than the 

 calculated velocity. The author has shown that steam is necessary for the burn- 

 ing of carbonic oxide, both with oxygen and nitrous oxide, and that as the pro- 

 portion of steam is increased the rate of inflammation is also increased. 



Preliminary experiments made in a lead tube. 55 metres long and 13 mm. 

 internal diameter, entirely confirmed MM. Berthelot and Vieille's experiments with 

 electrolytic gas. The veiocity of the explosive wave was found to be 2,817 metres 

 per second, as the mean of several closely concordant determinations at 10 c 0. 



"With a nearly dry mixture of carbonic oxide and oxygen, the explosive wave 

 was not established "until the flame had traversed a distance of 700 mm. from 

 the firinc point. The explosive wave was found to have a velocity of rather over 

 1,500 metres per second. After the explosion a fine layer of carbon was found to 

 cover the inside of the tube, showing that at the enormous temperature reached in 

 the explosive wave, carbonic oxide is decomposed into its constituents. 



G. On the Colour of Chemical Compounds. 1 

 By Professor Thos. Carnelley, D.Sc. 



The colour of chemical compounds is conditioned by at least three circum- 

 stances, viz. : — (1) Temperature (Ackroyd), (2) the quantity of- the electronega- 

 tive element present in a binary compound (Ackroyd), (3) the atomic weights of 

 the constituent elements of the compound (Carnelley), and that in such away that 

 the colour passes or tends to pass through the following chromatic scale : — 



"White or colourless Indigo Green Orange Brown 



Violet Blue Yellow Bed Black 



either by (1) rise of temperature, or (2) increase of the quantity of the electro- 

 negative element in a binary compound, or (3) with increase of the atomic weights 

 of the elements A, B, C, &c, in the compounds A x Br;, B x Bj,, C x R v , &c, in 



1 Phil Mag. (5), 18, 130. 



