6 OSMOTIC PRESSURE OF AQUEOUS SOLUTIONS. 



of the mixtures with water, and, finally, by a most thorough kneading 

 of the "putty." The results were still unsatisfactory in that the poros- 

 ity of the baked samples lacked the high degree of uniformity which is 

 indispensable in the measurement of osmotic pressure. It was evident, 

 moreover, on comparing our products with those of the potters, that 

 we had been trying exactly what they had attempted, except that, in 

 every case but one, they had omitted the elutriation and bolting pro- 

 cesses. It was concluded that the necessary binding material can not be 

 successfully incorporated with the clays in the form of ground feldspar. 



The final solution of the problem was easy and satisfactory. It 

 occurred to us that perhaps sufficiently intimate admixtures could be 

 obtained by bringing together two different clays, one of which is defi- 

 cient in binding material, while the other is over rich in that constituent. 

 This was the plan which was finally adopted, and with proper selection 

 and manipulation of the materials, it has never failed to give products 

 which are all that could be desired in respect to strength and uniformity 

 of texture. 



The pores were, however, still much too large, notwithstanding the 

 fineness of the materials, and the air blisters were not eradicated by the 

 usual method of forming such vessels. It was attempted to diminish 

 the size of the pores by repeatedly burning the cells at high temper- 

 atures, and in this way considerable but not sufficient improvement 

 was effected. Two plans had been proposed to the potters for securing 

 the required density of texture and for the simultaneous elimination of 

 the air blisters. The first of these was to form the cell itself under high 

 pressure, while the second was to form the wet clay into a cylinder under 

 great pressure, and from this to turn out the cell upon the lathe. Both 

 plans were declared to be impracticable by the potters. After many 

 months of futile effort, we were forced to agree with them as to the first 

 project, but the alternative plan — that of cutting the cell from a cylinder 

 which had been formed under high pressure — was finally developed to a 

 successful issue. 



TREATMENT OF THE CLAYS. 



A considerable number of clays, both American and foreign, were 

 investigated with reference to their suitability for the manufacture of 

 cells, and two were finally selected as being superior to any of the others 

 for the purpose. These were a fire clay from Dorsey,* Maryland, and a 

 so-called ball clay from Edgar, Florida. 



The Florida clay had been washed before it came into our hands, 

 while that from Maryland was in its original untreated condition. Two 

 processes have been employed for the separation of the finer portions of 

 the clays. Both give satisfactory products, but the earlier process has 

 been abandoned because the later one is more economical of material. 



*Erroneously stated to have been from Mount Savage, Maryland. 



