482 mi;, .i. n. i,\\vi:s, DB. GILBEBT, AND DB. PTJGH ox 



S. \ simple glass shade is liable to introduce fewer sourcei of error than a i 

 plicated metallic framework with panes of glass cemented into it Tin- shade is easier 

 to 'lean before commencing the experiment, and it is less likely to retain, at the tern • 

 nation of it, any of the combined Nitrogen, either derived from the plant, or from that 

 which lias been supplied during growth. Lastly, the presence of oxidizable metallic sur- 

 faces, affording a possible quantity of nascent hydrogen which might form ammonia with 

 the Nitrogen of the air, is avoided. 



4. There is no organic matter present which can affect the result of the cxperhn 

 The only organic matter within the shade is that of a thin coating of the gutta-percha 

 cement which has been described, by which the tube nl (fig. 1, Plate XIV.) i> fixed into 

 the hole ?i' (fig. 4, Plate XII.) at the bottom of the stone-ware lute. On analysis this 

 cement was found to contain from 0T0 to 0T5 per cent, of Nitrogen. Hence, if the whole 

 quantity of the cement in contact with the condensed water became decomposed, aud 

 yielded up its Nitrogen in such a manner as to become a product of the experiment, it 

 would only so yield a few tenths of a milligramme of Nitrogen ; but experiment proved 

 that it did not suffer sensible decomposition when subjected, during a whole year, to 

 exposure in the open air. 



5. In the passage of the air through the apparatus, the excess of pressure was upon 

 the inside, instead of, as in the experiments of others, upon the outside of the enclosing 

 vessel. In experiments of the kind in question in which the apparatus is exposed to 

 the open air, and so subjected to climatic vicissitudes during a considerable period of 

 time, the ordinary means of securing tightness in the laboratory cannot be depended 

 upon ; and an apparatus proved to be tight at one time may, as the result of a variety 

 of causes beyond our control, be subject to leakage at another. But a leakage from the 

 inside of the apparatus outwards cannot affect the result of our experiment ; whilst a 

 leakage in the opposite direction might introduce combined Nitrogen from the external 

 atmosphere. In the arrangement which has been described, the excess of pressure is 

 always on the inside during the passage of the air ; and when the air is not passing 

 there cannot be any important amount in the opposite direction due to changes of tem- 

 perature and barometric condition, for it can never exceed that required to drive the air 

 inwards through the bulb-apparatus M (Plate XIII., and fig. 1, Plate XIV.), which is 

 altogether insignificant. 



G. That part of the apparatus which would be the most liable to leak, and which 

 would be the most damaged by pressure, is subjected to the minimum amount of it. 

 The entire pressure required to force the air through the apparatus, independently of 

 that necessary to overcome friction, is 



5x1*85 = 9-25 inches of water 



to pass through the sulphuric acid in the bottles B and C (Plate XIII., and fig. 1, Plate 



XIV.), and 



2-5xT2 = 3-§ inches of water 



