Mr. Allen on the Volume of the Niagara River. 67 



of tribasic phosphate of soda, decomposing three equivalents of 

 nitrate of silver, producing one equivalent of tribasic phosphate 

 of silver, two of nitrate of soda, and one of nitrate of water ; this 

 last being liquid nitric acid, of course renders the liquor acid. 

 The reaction may be simply expressed 



P0 5 +2NaO.HO and 3(NO*+AgO) 

 give P0 5 +3AgO . . . 2(N0 5 +NaO) and N0 5 +HO. 

 If on the other hand, bibasic phosphate of soda be used, the liquor 

 remains neutral, for P0 5 +2NaO and 2(N0 5 +AgO) give P0 5 -f 

 2AgO and 2(NO s +NaO). 



"In the tribasic phosphates, it frequently occurs, that there shall 

 be but one equivalent of fixed base, the other two being water ; 

 such salts have frequently an acid reaction, and were formerly 

 called biphosphates. Thus one tribasic phosphate of soda is 

 PO 5 -f Na0.2HO ; the biphosphate of ammonia is tribasic, its for- 

 mula being P0 5 +NH 4 0.2HO. 



" These salts of phosphoric acid were originally designated by 

 Graham, metaphosphates, pyrophosphates, and common phos- 

 phates." 



Art. VII. — On the Volume of the Niagara River, as deduced 

 from measurements made in 1841 by Mr. E. R. Blackwell, 

 and calculated by Z. Allen. 



Yery little attention appears to have been hitherto bestowed on 

 the investigation of the comparative volumes of water discharged 

 by the great rivers of the globe. The relative amount of the 

 evaporation and drainage from the soils of different countries in 

 proportion to the quantity of rain that falls upon each, as denoted 

 by rain-gauges, is also another interesting subject connected with 

 the preceding one ; for by measuring the quantity of water dis- 

 charged from a region of country by the streams that drain it, and 

 by deducting this quantity from the whole amount that falls upon 

 it as indicated by rain-gauges, the relative amount of evaporation 

 may be ascertained. The investigation of these facts forms the 

 basis of a branch of the science of hydrography, and leads to many 

 useful as well as curious and interesting enquiries. 



Whilst passing a few days at the Falls of Niagara, in the sum- 

 mer of 1841, it occurred to me to make the necessary admeasure- 



