190 DR. R. ANGUS SMITH 



bottle corresponding to the amount of carbonic acid^ and 

 tbe trial could be made at once. This plan would not 

 suffice for estimating the amount in any given air ; it would 

 estimate only one amount ; but it would show clearly when 

 there was more and when less. 



When it was found so easy to remember a certain bulk 

 of precipitate^ it became important to know what bulk 

 would be the most easily remembered. Must it be a 

 minute quantity^ such as a chemist would call a trace, or 

 must it be a quantity such as we should call milky ? Neither 

 suffice. The first is too smaU for certainty ; the second 

 has no translucency, or so little that we cannot judge of 

 the amount that lies behind. The quantity will be expressed 

 most clearly by saying that the liquid is turbid and still 

 translucent ; but not so that you could read through it. 

 Any one may obtain it exactly by shaking a clear 23-ounce 

 bottle with half an ounce of baryta-water in air containing 

 0*04 per cent, carbonic acid ; and this may easily and fre- 

 quently be done to aid the memory. To be more precise,, 

 it is a precipitate obtained by throwing down baryta with 

 0*2515 cub. centim. of carbonic acid, or 0*00224 gramme 

 carbonate of baryta freshly precipitated in half an ounce of 

 liquid. In Table I. all the information actually necessary 

 is given. Column 2 is for fine measurements in cubic cen- 

 timetres, indicating the amount of air which will contain . 

 the carbonic acid necessary for producing the precipitate 

 of baryta when the proportion is according to any number 

 in the first column. Column 3 is the same number, with 

 the addition of 14' 16 cub. centims., or half an ounce, which 

 is the space occupied by the liquid. This, then, gives the 

 size of the bottle to be used. The fourth column also gives 

 the size of bottle to be used, the numbers being avoirdupois 

 ounces ; fractions are not in all cases given, and are not 

 required so minutely as they are given in some. 



