Chemical Society. 329 



rous acids, free chlorine, chloride of iron and sulphate of soda ; these 

 arise from impurities contained in the materials employed ; the 

 chlorine from the action of nitric or nitrous acid (often present in 

 oil of vitriol) on the hydrochloric acid ; sulphurous acid from or- 

 ganic particles in the common salt employed ; and chloride of iron 

 from the presence of that metal also in the salt. Pure and clean 

 materials are therefore the first requisite for a pure acid. Dr. Gre- 

 gory finds, that if to one equivalent of salt two equivalents of sulphu- 

 ric acid diluted with a certain quantity of water be used, instead of 

 one equivalent as usually prescribed, the whole of the hydrochloric 

 acid may be expelled without a trace of sulphuric acid passing over 

 even into the first condensing bottle, and that two-thirds of the hy- 

 drochloric acid distil over before water is volatilized ; on this obser- 

 vation the following process is founded. 



Into a common Florence flask are introduced 4 ounces of the pu- 

 rest patent salt, and 5 fluid ounces of sulphuric acid of specific gra- 

 vity 1*600 ; a gentle heat is applied, and the gas which is then gene- 

 rated, is conducted by a bent glass tube into a four- ounce phial con- 

 taining 2 ounces of distilled water surrounded with snow or ice-cold 

 water. No safety tube is required, as the tube is made to dip only 

 about one-eighth of an inch into the water, so that should any 

 absorption take place the rise of a little water in the tube exposes 

 the extremity of it, so as to admit the air ; or, for greater security, 

 a small bulb may be blown on the descending limb of the tube. 

 The gas is absorbed as fast as it comes over, and for the first hour 

 and one quarter the heat hardly requires to be increased ; if the tem- 

 perature of the surrounding water has been kept so low as 50°, the 

 2 ounces of distilled water will have increased in volume to 3 ounces 

 of colourless hydrochloric acid, fuming strongly, and having a spe- 

 cific gravity of 1*20 to 1*21, the gas passing over so dry that no part 

 of the tube becomes warm. This portion being removed, its place is 

 supplied by 2 ounces more of distilled water, and the heat gradually 

 increased and continued for an hour longer; by that time all the hydro- 

 chloric acid is expelled, with some water, and the 2 ounces of water 

 have become 3 ounces of hydrochloric acid of specific gravity 1*10. 

 Both portions are absolutely pure. If 3 ounces of water are used in 

 the first instance, 4*5 fluid ounces of acid of specific gravity 1*165 

 are obtained ; and then replacing the acid by 2 ounces of water, 3*5 

 ounces more of specific gravity 1*065. If 5 ounces of water are used 

 at once for condensing the acid and kept till the distillation is com- 

 plete, 7*5 fluid ounces of specific gravity 1*155 are obtained. Dr. 

 Clark finds sulphuric acid of a specific gravity of 1*65 to answer 

 still better than acid of 1*60. 



Dr. Clark then exhibited to the Society his method of ascertain- 

 ing, quantitatively, the comparative hardness of water by means of 

 the common test of tincture of soap, illustrated by experimental 

 evidence, to prove the accuracy of which it was susceptible, and the 

 facility of its application. Dr. Clark hoped at a future meeting 

 to lay before the Society more mature details of the method ex- 

 hibited. 



