HAP. II.] LABORATORY WORK ON GASTRIC DIGESTION. 179 



[The following is a reprint of the directions for experiments on the 

 detection of small quantities of acids, and on the action of acid, alkali and 

 peptone in modifying the action of "ptValin, drawn up by Mr J. N. Langley, 

 F.R.S., for the use of students attending his advanced Practical Course 

 in the University of Cambridge: 



" 2. Detection of small amounts of free HCl. Tropaeolin OO is 

 changed in colour much more rapidly by free acids than by acid combina- 

 tions, so that whilst small quantities of either give an acid reaction with 

 litmus, an acid reaction with tropaeolin OO is given by the former only. 



" (a) The most delicate method of detecting small quantities of free 

 acid is the following : a saturated solution of tropaeolin OO is made in 

 strong spirit, a drop of this is evaporated on a porcelain plate which is 

 placed in a chamber at about 40 C., and to the dried tropaeolin is added 

 a small drop of the fluid supposed to contain free acid ; at once, or as the 

 luid evaporates, the yellow tropaeolin becomes of a violet or purple tint if 

 acid is present. HCl 0*005 p.c. can thus be detected. 

 " (b) When the percentage of HCl is somewhat greater than this, it is 

 ipler to pour a little of the solution into a test tube, containing a small 

 lount of a saturated aqueous solution of tropaeolin OO, when the yellow 

 )lour is changed to orange. About OO1 p.c. HCl and 0*06 p.c. lactic acid 

 in thus be detected : the colour produced by lactic acid is removed by 

 ler, that produced by HCl is not, unless the amount of HCl be very 

 11. 



" (c) Paper soaked in tropaeolin OO and dried becomes brownish and, 

 drying, lilac or violet when a drop containing free acid is placed on it. 

 " 3. Methyl- violet is also used by method (b) to detect small quanti- 

 of free acid, this colour is changed to blue in the presence of not less 

 0-05 p.c. of HCl or 0'5 p.c. of lactic acid. 



" 4. To a few c.c. of O'Ol p.c. of HCl, a drop of which gives a bright 

 violet mark with tropaeolin OO (method a), add a little peptone, the 

 mixture gives no acid reaction with tropaeolin ; a similar result is pro- 

 duced by adding serum, or white of egg, probably in consequence of the 

 globulin in these fluids ; myosin and certain other proteids act in the same 

 manner. 



" 5. Take 10 c.c. of 1 p.c. solution of peptone, it will probably be 

 acid to litmus ; if so, neutralise it with i^COg, noting the volume added. 

 Add HCl, O'lp.c., from a burette, until a drop of the mixture gives a 

 violet mark with tropaeolin OO ; the mixture then contains 0'005 p.c. of 

 free HCl. The amount of HCl added, minus the amount of free HCl, 

 gives the amount of the acid which has combined with the peptone ; from 

 this, the percentage of HCl taken up by the peptone should be calculated 

 thus: suppose 5 c.c. of HCl (! p.c.) is the amount taken up by the peptone, 



. 5 x -1 100 K 

 then the percentage is _- ^ * = = 5. 



"6. Effect of acid, alkali, and peptone slightly acid, on ptyalin. 



" The ptyalin solution may be prepared thus : 



" (a) Chop up a ptyalin containing salivary gland, e.g. the parotid 

 of a rabbit, add about 100 c.c. of water, leave at 39 C. for an hour or two. 

 Filter, neutralise, and, if necessary, filter again, (b) Neutralise freshly 

 collected saliva, dilute 10 times and filter. 



122 



