Cadmium, and Mercury. 371 



tions in the field alluded to contain more mercuric chloride 

 than this ratio. The chemical reaction which will be proved 

 below is : 



2H 2 + 3HgCl 2 4-2Na 2 S 2 3 =HgCl 2 . 2HgS+4NaCl + 2H 2 S0 4 



2. All solutions lying in the field included between I, II and 

 I,III precipitate mixtures of HgCl„.2HgS and black HgS, 

 apparently amorphous. All solutions on the line I,III contain 

 HgCl 2 and JN T a„S„0 3 in the ratio 1 mol : 1 mol. The chemical 

 reaction is H.O + Na„S„0, + HgCh = 2NaCl + H 2 S0 4 + HgS 

 (black). 



3. All solutions lying between the lines I,III and I,IV precipi- 

 tate, in the beginning, black HgS, though later a red precipi- 

 tate may be obtained. 



4. The line 1,1 V includes all solutions in which the ratio of 

 HgCl„ to Na 2 S 2 3 is 1 mol : 4 mol. The principal reaction is 

 HgCl" 2 + 4Na 2 S 2 3 = 2NaCl + 3Na 2 S0 4 + 4S + HgS. The 

 mercuric sulphide precipitated here is, in the beginning, a deep 

 red modification which the microscope shows is not cinnabar 

 but a new form, appearing transparent and orange-colored under 

 the microscope and having lower indices of refraction than 

 cinnabar. It should be stated here that the precipitation of 

 this phase continues until the dilution of the supernatant liquid 

 has fallen to about 1 per cent of mercuric chloride, when the 

 black form in crystalline condition begins to come down with it. 



Proof of the above reactions. — 1. The reaction 3HgCl 2 + 

 2Na 2 S 2 O s + 2H 2 = HgCl 2 . 2HgS + 4NaCl + 2H 2 S0 4 . A 

 solution containing 2 - g. NaCI, 1"64 g. HgCh and 1*000 g. 

 Na 2 S 2 3 . 5H„0 in 100 to 200 cc water was boiled for a short 

 time and filtered. No gas escaped during the process. The 

 curdy yellowish precipitate was carefully washed, and the fil- 

 trate and washings were diluted to 250 cc . This solution was 

 free from mercury, gave a strong acid reaction with litmus and 

 a decided reaction for sulphate with barium chloride. 



0-2011 g. pure dry sodium carbonate required of this solution 

 120*6 CC for neutralization, using methvl orange as an indicator. 

 0-386 g. H 2 S0 4 found. 0-394 g. H 2 S0 4 cah from the above 

 equation.* In a second experiment 20 g. NaCl, 1*65 g. HgCl 2 

 and 1-000 g. JS"a 2 S 2 3 .5H 2 in about 200 cc water were boiled 

 and filtered as before. f The filtrate was diluted to 500 cc . It 

 contained no mercury, no thiosulphate and was strongly acid 

 as before. No sulphur dioxide was evolved during the reaction. 

 0-3006 g. 1N\C0 3 required for neutralization 348'5 ce solution, 

 using as indicator phenol phthalein at boiling temperature. 

 Total H 2 S0 4 found 0-3991. Cal. from equation -3955. Thor- 



* It will be remembered that the thiosulphate has the formula Na 2 S 2 3 . 

 5H 2 0. 

 fit may be the slight excess of mercury was absorbed by the precipitate. 



