SCIENCE. 



the weaker ones in order to effect the disappearance, a 

 greater quantity of the iodine present in them would have 

 been converted into hydriodic acid, and this would tend to 

 an equalization of the amounts of iodine present on cooling 

 in the various cases. Owing to this conversion of iodine 

 into hydriodic acid, the solutions on cooling, as might be 

 expected, are considerably lighter than they were before 

 heating, and their intensity naturally depends to a great 

 extent on the rapidity with which thev have been cooled ; 

 even a very weak iodine-starch solution which has been 

 heated may be made to re-assume its color if cooled 

 very quickly. 



The amount of starch which may be recognized by 

 means of the iodine reaction varies, of course, with the 

 bulk of liquid operated upon. Using about 200 c.c. the 

 weakest solution which gives an easily discernable blue 

 tint in a beaker contains about 0.0001 per cent, of starch, 

 while if small quantities are examined in a test-tube this 

 percentage must be doubled in order that the color may 

 be rendered visible. The green color which is noticed 

 when a large quantity of iodine is added to a weak solu- 

 tion of starch, appears to be due simply to the combina- 

 tion of the proper yellow color of the free iodine with the 

 blue color of the iodine-starch. 



When two weak solutions of iodine, to one of which 

 some starch has been added, are exposed to the air in an 

 uncovered beaker, the iodine in both cases disappears en- 

 tirely in the course of a few days, but more slowly from 

 the solution which contains the starch ; hence the iodine 

 which disappears (owing partially to its volatilization into 

 the air and partially to its hydrogenation) seems to be 

 retained to a certain extent by the presence of starch. 

 The presence of iodine has a reciprocal action in the pre- 

 servation of starch. A solution of starch, which, in a few 

 days, is converted into dextrin, may be preserved unal- 

 tered for a lone time — possibly for an indefinite time, if 

 an excess of iodine is present in it. 



When a sufficient quantity of iodine is added to a solu- 

 tion of dextrin, a deep brown color is produced ; the col- 

 ored compound which is here present is in a state of true 

 solution, whereas in the case of starch it will, as is well 

 known, settle entirely to the bottom of the liquid in deep 

 blue flocks, leaving the supernatant solution quite color- 

 less, and these flocks on agitation are disseminated again 

 so as to form an apparent solution. The dextrin reac- 

 tion with iodine is not nearly so delicate as that of 

 starch ; the weakest solution which gave any discernable 

 color on being tested contained 0.005 P er cent, of dextrin, 

 and in this case the color could only be detected by 

 using about 200 c.c. of the solution, and comparing the 

 color with that of some iodine solution of the same 

 strength as that to which the dextrin had been added. 



With starch, the first drop of iodine which is added 

 produces a permanent coloration. With dextrin, how- 

 ever, this is not the case ; the color produced by the first 

 drops disappears instantly and entirely. A considerable 

 quantity must be added before a moderately permanent 

 color is produced, and the reaction, owing to which the 

 iodine disappears in this way. will continue for six or 

 seven days. Whether the dextrin disappears or not at 

 the same time has not been ascertained, although it 

 seems most probable that it should do so. 



When a solution of iodine-dextrin is heated, the color 

 becomes lighter and gradually disappears, as in the case 

 of iodine-starch, but the temperature at which this disap- 

 pearance takes place is considerably lower. An opaque 

 brown solution on being heated in a flask became color- 

 less at about 8i° C, and, on cooling, regained its color 

 with considerable diminution in intensity) at 64° C. A 

 solution of one-quarter the strength of the preceding one 

 lost its color at 52 , and regained it on cooling at 34 C; 

 here also, as in the case of iodine-starch, we find that 

 the colored principle does not become colorless at any 

 particular temperature, but its disappearance is dependent 

 on its original intensity, 



The dextrin usually met with in commerce contains a 

 considerable amount of starch, which, however, may be 

 entirely converted into dextrin by prolonged heating at 

 140 to 160 C for several hours, after which it gives the 

 pure brown reaction with iodine above mentioned. 



When iodine is added in excess to a mixture of starch 

 and dextrin, the colors produced are blue, violet, purple, 

 claret, red-brown, or brown, according to the various 

 proportions in which the two substances are present. 

 When the iodine is added gradually to the mixed solu- 

 tions the colors produced, both temporary and perma- 

 nent, follow the same order as those above mentioned, 

 the blue colors appearing first, and the red ones only on the 

 addition of larger amounts of iodine. Conversely, when 

 the colored solution is allowed to stand, the red tints 

 disappear first, and the blue ones last. Obviously, 

 therefore, the gradual addition of iodine affords an easy 

 and delicate means of detecting starch in the presence of 

 even a large amount of dextrin. Another way in which 

 starch may be detected in similar cases, is to add an 

 ample sufficiency of iodine to produce a permanent color, 

 and then to heat the liquid ; the brown iodine-dextrin is 

 decomposed at a comparatively low temperature, while 

 the blue iodine-starch remains till the heat is raised con- 

 siderably higher, and again, on cooling, the blue tint re- 

 appears long before the brown or red tint does ; even 

 when there' is not sufficient starch to yield satisfactory 

 results by this method, it may often be detected by the 

 liquid being of a more bluish tint after the heating than 

 it was before it. 



O. Knab (Chem. Centr. Blatt, 1872,492) found that 

 some dextrin which he had prepared by repeated (ten 

 times) precipitation with alcohol gave the reaction of a 

 mixture of dextrin and starch, and hence concluded that 

 it still contained some of this latter substance. It ap- 

 pears superfluous, however, to raise an impure prepara- 

 tion to the dignity of a chemical compound by giving it a 

 distinct name — dextrin-starch — as Knab does. On leav- 

 ing a mixture of solutions of starch and dextrin for some 

 days, Knab found that, whereas the addition of iodine had 

 at first caused a deep blue coloration, after a time noth- 

 ing but the red or brown color of iodine-dextrin was pro- 

 duced, and hence draws the somewhat startling conclu- 

 sion that starch under the influence of dextrin is converted 

 into dextrin. A simpler and more probable conclusion 

 from these experiments would surely have been, that at 

 the end of the few days during which his experiments 

 lasted, the starch had suffered that spontaneous decom- 

 position to which it is, as is well known, so prone, leav- 

 ing in solution nothing which would give a coloration 

 with iodine but the unaltered dextrin. 



Dextrin and starch, it appears, give entirely different re- 

 actions with iodine ; the former combines with the halo- 

 gen to form a brown soluble substance, whereas the lat- 

 ter forms with it a deep blue insoluble body ; and these 

 two reactions are so distinct that presence of either of the 

 reagents may be easily detected in a solution containing 

 both of them. 



The fact that the addition of iodine to dextrin produces 

 only a transitory color at first, and that an excess of it is 

 necessary to give a permanent tint, will, no doubt, ex- 

 plain the various discordant statements which exist as to 

 whether any color is produced by the mixture of these 

 two substances or not, and will probably render unneces- 

 sary the theory of there being two or three different dex- 

 trins, as proposed by Mulder and Griessmayer. 



Determination of the Fatal Dose of Carbonic Oxide 

 for Various Animals. — Air containing 1.300th of its vol- 

 ume of carbonic oxide proved fatal to a dog when inhaled 

 for fifty minutes. In another dog of the same size the 

 fatal dose was i-250th. A rabbit resisted various propor- 

 tions up to i-6oth. A sparrow perished with l-50oth, — ■ 

 M. Grehant. 



