2 4 0 Scientific Notes and Comments. [ J. F. I.,

C H E M I S T R Y .

ON THE CARBONIC ACID CONTENT OF THE ATMOSPrtERm AIR. By R, Blochmann(Ann. 2 3 7 , 3 9 - 9 o ) . ' T h e above paper combines a thorough t om - parat ive study of the methods employed and the experiments performed up to the present time, for the purpose of de te rmining the amount of carbonic acid contained in the air, with an account of the modification and improve- men t by the author of the Dal ton-Pet tenkofer method for de termining atmos- pheric carbonic acid. Considerable space is devoted to the discussion of the sources of error, to which the various methods heretofore employed are sub- ject, and at tent ion is called to the fact tha t as more refined methods came into use, and more care was taken to determine and el iminate errors of experi- ment , the srrialler the accepted figure for the amount of CO 2 in the air became. Thus :

PRR~ODS.

I828 to I83o

i83o " 1856 x856 ,, x885

NUMBER OF--

!

ob (S.u.~.ro) . . . . . . . . . . 6' I1> Ob . . . . . ,.o~o 3.~ .o. ,i . . . .

Vol. C02 in xo,ooo Vo|.

Air.

4"1[

3"6 3"x

Of all the methods hi therto used for de te rmining the carbon dioxide of the air, the so-called Pettenkofer method, on account of its simplicity and rapidity, was chosen by the author as the one whose imperfections could be dealt with with the greatest hope of success. T he principle of the method is this, viz., into a large glass bottle conta in ing . the air to be examined, is run a measured, quanti ty of s tandard bary ta water. The bottle is then corked and shaken , and the bary ta solution t ransferred to a suitable vessel, t i trated with s tandard acid, and the excess determined. T he principal sources of error are, first, the absorpt ion of CO2 by the bary ta solution from the air of the room during the titration. Second, in washing the bottle with hot water, more or less decomposit ion of the glass occurs, render ing the solution more alkal in ,e~ Third, in the use of caoutchouc stoppers, which, in accordance with numerous observations, when in contact with a lkal ine liquids, undergo oxidation, with the liberation of carbon dioxide. The first of these difficulties has been met by the employment of a compound burette, communicat ion between the two halves of which is effected by means of a three-way glass stop-cock. In this piece of apparatus, both the acid and the bary ta solution to be determined, may be measured and ti trated out of contact with the air, with the greatest accuracy. The second of the above operations has been rendered unnecessary. In order to avoid the use of caoutchouc, the neck of the bottle is fitted with a stopper, consist ing of a well-fitting glass-plate, a layer of mercury, and then a layer of paraffin, the whole be ing bound down by a piece of sheet-rubber. Through the stopper pass two glass-tubes with stout walls, and supplied with glass stop-cocks. One of the tubes reaches to the bottom of the battle,

Mar., I887.] Scientific'Notes and Comments. 241

The sum of the constant errors affecting any result by this method is '1o and the probable error of a single experiment lies between + o'o2 5. The following table contains estimations of the carbon dioxide in the air from the laboratory garden, KSnigsberg :

DATES.

I885, September 4 5

" " 9 " " i o

,c '" i i , , cc 1 2

~, . 1 3

Bottle No. 64. Vol. CO2 in xo coo Vol.

Air.

I l I Mean.

3"09 3"iz 3.]o 3~4 3'24 3 24 3'I0 3'~3 3'If 3'09 3 08 3'08 3"*9 3"z5 3"17 3"*5 3 "I8 3'z7 2 05 [ 3'o0 2 97 2"9r 296 2'93 3 u6 ' 3 I'3 3"4 3"06 3"o9 3"07

y*o

Bottle No. 95- [ Vol. CO2 in io, ooo Vol. I

Air. Joint Mean.

I

306 3"25 3"H 3'07 3*9 3u5

"98 ~94 3']4 3'o3

l I Mean.

3'1~ 3 .08 3'26 3"~5 3'*z 3"~i 3'°9 3 08 3":8 3"i8 [ 3"-W 3 I6 ] 2"98 2'98 2 " 9 5 2 " 9 5 [ 3"x5 3'*5 ] 3'05 3"o5 /

. . . . /

3'*0 [

• 3'o9 3"25 3"n 3"o8 3"x8 3'x6 2"98 2'94 3"*5 3"°6

3"Io

A. G. P.

A N E W R E A C T I O N F O R T H E D E T E C T I O N O F S M A L L Q U A N T I T I E S O F H Y D R O -

CYANIC ACID.--G. Vortmann (Mona[schefte, 7, 416,) states that Playfair's reaction of the cyanides and nitrites is an exceedingly delicate test for a cyanide or hydrocyanic acid, the resulting nitro-prusside being recognized by its reaction with sulphides. The liquid to be tested is mixed with a few drops of potassium nitrate solution, from two to four drops of ferric chloride solution, and enough dilute sulphuric acid to change to bright yellow the first formed yellow-brown color of the basic iron salt. The mixture is heated until it begins to boil, then allowed to cool, and after the separation of the excess of iron a few drops of ammonia are added; the liquid is filtered and the filtrate treated with one or two drops of a much-diluted solution of colorless ammonium sulphide. The presence of hydrocyanic acid in the liquid will now occasion a beautiful violet color, changing in a few minutes to blue, then to green and yellow. Traces of hydrocyanic acid occasion only a bluish- green color, that rapidly becomes yellowish. The limit of the reaction is put at a dilution of I :312,5oo, ten cubic centimetres of liquid being used; the limit of the Prussian blue reaction is I : 5o,ooo; that of the ammonium rhodanate I :4,000,00o. W . H . G .

A NEW EXPLOSIVE MIXTURE.--A. Cavazzi (Gazzetta Chimica ltaliana), in studying the reduction of potassium nitrate by various substances, has found that a mixture of equal parts of the nitrate and sodium hypophosphite detonates violently when heated to about the fusing point of the mixture. The experiment should be made on small quantities only, 'and while other proportions yield an explosive mixture, those mentioned are the best.

• W . H . G .

D E T E R M I N A T I O N O F F A T I N M I L K , ETC. M . K r e t z s c h m a r . (Chem. Z e i -

lung. I886. IOO.)--A gravimetric determination of fat in milk and other