1821 '.] Strontiariy Lime, Magnesia, Phosphoric Acid, S^c. 9* 



contained any barytes nor any sulphuric acid in solution ; sO! 

 that the sulphuric acid in the 7*5 grs. of sulphate of magnesia 

 had just saturated the barytes from 13-25 grs. of chloride of^ 

 barium. It is obvious then that the true weight of an atom of 

 mao-nesia is 2-5 ; therefore, the atomic weijght assigned by 

 Dafton and Wollaston is too small, while that assigned by Ber- 

 zelius is too high. Accordingly, if we mix together anhydrous 

 sulphate of magnesia and chloride of barium in the proportions 

 indicated by these numbers, we shall in the one case find an 

 excess of sulphuric acid, and in the other of barytes, in the hquid 

 after the precipitate has subsided, indicating obviously an error 

 in the weight of the salts thus mixed together, and consequently 

 an error in the numbers assigned by these gentlemen for the 

 weight of an atom of magnesia. 



4. The weight of an atom of phosphoric acid has cost me^ 

 first and last a good deal of trouble. I have the happiness, 

 however, at last to be able to lay before the reader experiments 

 of so decisive a nature that no doubt nor uncertainty can rest 

 upon the subject for the future. In the year 1816 I drew up a 

 paper upon the subject, the result of a good many experiments, 

 which was read before the Royal Society. Some discussion, 

 took place in the committee of papers relative to these experi-r 

 ments ; and Dr. Wollaston, who was a member of that commit?-? 

 tee, and to whose friendship and assistance I have been 

 very frequently obliged, kindly brought the paper to me to give 

 me an opportunity of correcting some numerical mistakes which 

 ne had observed in it. By this time I had made the experiments 

 on phosphuretted hydrogen gas, which were soon after published 

 in the Annals of Philosophy. These experiments had made nae 

 acquainted with the true weight of the atom of phosphorus, 

 phosphorous acid, and phosphoric acid, and had explained all 

 the errors into which I had fallen in my original paper. I had, 

 therefore, been extremely desirous of withdrawing my paper 

 from the Royal Society, in order to have an opportunity of cor- 

 recting it. Of course, when it was put into my hands by 

 Dr. Wollaston, I requested of the Society to be allowed to keep 

 it, and this request they were kind enough to indulge me in. 



Just at the time that my proof sheet giving an account of 

 phosphorus in the fifth edition of my System of Chemistry was 

 in my possession, 1 received Mr. Dalton's short paper on phos- 

 phuretted hydrogen gas, which was printed in the Annals of 

 Philosophy, In that paper Mr. Dalton states that phosphuretted 

 hydrogen gas is capable of condensing twice its volume of 

 oxygen gas. I had just before been informed by Gay-Lussac of 

 Dulong's discovery of hypophosphorous acid, and had read over' 

 Berzelius's paper on phosphorus and its compounds, in which he • 

 shows by a number of analyses agreeing very well with each- 

 other that the atomic weight of phosphoric acid is 4*5, or at* 

 least very near that number. Being perfectly sure of the accu- 



