T. M. Chatard— Urao. 63 



These small differences show that each of the samples is urao. 

 In each case there is a varying amount of other salts and impu- 

 rities to be deducted, but when this is done the residue, in four 

 out of the five samples, shows a very close agreement with the 

 formula of the mineral. In the case of No. 4 or the " Lagune " 

 the differences are quite large, but as the local conditions attend- 

 ing the production of each specimen are well known, the expla- 

 nation is simple. Unlike the others which are products of un- 

 disturbed crystallization, this one is apparently the result of an 

 interchanging concentration and dilution of the mother liquor 

 in which it was formed. As the water in such a shallow basin 

 evaporates the tendency is to leave a crust of very pure urao, 

 at the edge of the basin, the deposit towards the center 

 becoming more and more impure as the concentrating liquid 

 deposits its chloride and sulphate and becomes, as experiments 

 show, a comparatively pure solution of sodium mono-carbonate. 

 If then the basin be refilled by seepage, as would be the case 

 when the lake rises in the spring, the solution would contain a 

 larger proportion of the neutral carbonate and, on reconcentra- 

 tion, would leave on its edges a urao containing an excess of 

 hydrated monocarbonate. If we calculate the excess of mono- 

 carbonate and water present, we shall find that the two combine 

 to form Na 2 CO s +2H 2 and that the sample represents 84*71^ 

 urao, 12-06^ Na 2 C0 3 , 2H 2 0, -02^ H 3 and 2*21# impurities. 



Artificial Urao. 



A series of experiments was undertaken, in order to deter- 

 mine the conditions under which urao is formed and also to 

 find out if, by spontaneous evaporation, under known condi- 

 tions the sesquicarbonate or any other combination of mono- 

 and bicarbonate, other than urao, might be formed. For this 

 investigation a number of solutions was prepared, each of which 

 contained Na 2 CO a , NaHC0 3 and NaCl, the amount of each salt 

 employed having a certain definite relation to its molecular 

 weight. NaCl was added because its presence appears to exer- 

 cise a favorable influence on the crystallization of the mixed 

 carbonates. A full account of these experiments and results 

 will be found in the Bulletin from which this paper is con- 

 densed ; for the present it is sufficient to say that in no case, no 

 matter what the relative proportions of the salts might be, was any 

 other mixed carbonate but urao obtained. If the ]STaHC0 3 

 was present in excess a portion crystallized out first, as such, 

 but this was invariably followed by crystallizations of urao. 

 On the other hand if the Na 2 C0 3 was in sufficient excess, the 

 urao first obtained was contaminated with the former salt. 

 The following examples will show this. The solutions were 

 made up as follows : 



