458 0. PIAZZI SMYTH ON MICROMETRICAL MEASURES OF 



APPENDIX III. 



MR CHARLES F. CASELLA'S LETTERS ON THE PREPARA T ION AND 

 PURIFICATION OF SOME OF HIS LATER VACUUM TUBES. 



147 Holborn, London, E.G., 



31st August 1883. 

 To Professor C. Piazzi Smyth, F.R.S.E, 



15 Royal Terrace, Edinburgh. 



Dear Sir, — I am in due receipt of your favour of the 30th inst., and I must thank you 

 for the kind expressions that you make use of with regard to my Father, as well as for your 

 kindness in communicating with me on the subject of the Tubes. 



By train to-day I have great pleasure in sending you three tubes, viz., the 4th CO + CO 2 

 Tube at 0-2" pressure, and the CO + CO 2 Tube at 0-3" pressure, also the N tube at 12'5" 

 pressure — all of which I trust will be entirely to your satisfaction. 



This now completes all the Tubes I have had to do for you, and I now beg to give you a 

 formal note of how the gases in the above tubes have been made, the various processes being 

 as follows. 



Hydrogen, by electrolysis of water. Oxygen I tried as above, but the manufacture of the 

 gas was so dreadfully slow, that I had to resort to a chemical process, viz., by heating chlorate 

 of Potass, which I think is the most satisfactory way of obtaining Oxygen. 



Nitrogen, by boiling Ammonium Nitrate ; of course the preliminary bubblings yielding 

 impure Nitrogen, were allowed to escape, and only the subsequent bubbles of gas collected. 

 Carbonic Acid, by heating ferro-cyauide of Potassium with eight times its weight of 

 sulphuric acid. 



Coal-gas simply by connecting the gas jet with the gas receiver connected to the Pump. 



CO-f-CO 2 prepared by heating crystallised oxalic acid with concentrated sulphuric acid. 



All the above gases were prepared in glass retorts, then passed through water into glass 

 receiver, which latter was connected with the Pump by glass tubes, and a very delicate small 

 steel tap; the various drying tubes used were — next to the end-on tube a Caustic Potass tube; 

 immediately next to which was an Anhydrous Phosphoric acid tube : then came a four-foot 

 pummice-stone tube saturated with concentrated sulphuric acid ; next to which was a small 

 chloride of Calcium tube, and then came the small steel tap separating the Pump from the gas 

 receiver. Before making each gas the trough, receiver, and everything were thoroughly 

 cleaned, and fresh water and new drying tubes used each time ; a new Phosphoric Acid tube 

 being used for each tube. 



I trust I have made myself clear in all the above details, but if I have not, pray do not 

 hesitate to ask me for further particulars. 



Assuring you of best attention at all times, and hoping that at some time I may have the 

 honour of being specially mentioned in connection with the preparation of these tubes, which 



