SCIENTIFIC SUMMARY. 
367 
red.’ From the tannic acid, by heating it with acids in closed tubes, only 
gallic acid as well as ‘ oak-red ’ were obtained. If this operation be performed 
with hydrogen chloride, the development of a gas, w T hich burns with a green 
flame (methyl chloride), is observed. The dry distillation of tannic acid 
yielded a small amount of brenzcatechin and a mixture of oily products, in 
which, very probably, the presence of dimethylbrenzcatechin was remarked. 
When fused with potassium hydrate there are formed, as Grabowski has 
shown, protocatechic acid, brenzcatechin, and phloroglucin. Digested with 
emulsion, or boiled with dilute acid, it yields no sugar-like bodies. It cannot, 
however, be regarded as glycoside — a conclusion which will already have been 
arrived at, from the numbers indicating an anhydride. ( Wien Anz. 1880, 61.) 
The Passivity of Iron . — If a bright piece of iron be dipped into nitric 
acid of 42° B, a brisk effervescence of gas takes place ; this soon suddenly 
stops, and the metal appears bright and lustrous, and remains so. This occurs 
also when more dilute acid is used, down to 35° B. With the latter, also, 
passivity sets in after a certain time, but it does not last, and after a certain 
time the action begins again at a point of the surface, to suddenly stop again 
later on. If an iron rod be partly dipped into strong acid between 42° and 
35° B, and then it be brought, with very great care, into more dilute acid, the 
already moistened part first, and then the upper part of the rod is slowly 
introduced, no action takes place at first ; after some time some bubbles are 
seen to form on the metal, which soon suddenly cease to be developed, the 
rod becomes passive, and has become so because the rod had been dipped 
partly into strong acid. This condition has, however, no great stability ; a 
slight shaking is sufficient to bring about a change. The degree of pas- 
sivity produced in this manner is variable, and, in fact, is the smaller, the 
more dilute the second acid, the rougher the surface of the metal, and the 
greater the diameter of the rod. As soon as the metal is brought into a 
concentrated acid chemical action takes place. The bubbles of gas generated 
by the action are at first readily dislodged from the metal and are taken up 
by the surrounding liquid. This solubility, however, is limited, and the 
bubbles of gas remain adhering by capillary attraction to the metal, forming 
a protecting coat, which, however, can be dislodged by shaking. If the 
acid, however, is less concentrated — say 36° B. — the phenomenon is intermit- 
tent, because the coating is dissolved after some time, and a new one will 
then be formed. In the experiments with the partly immersed iron, the 
action of the adhering film of gas is propagated by the slow immersion of 
the iron slowly upwards, and exercises an attraction on the newly-formed 
bubbles, whereby the coating is continued from one part to another. In 
order to determine the influence of the dilution of the acid on the duration of 
the passivity, the author, L. Yarenne ( Compt . Rendus, 1880, xc. 998), made 
specimens of iron passive, and introduced them into acids of different degrees 
of dilution, as below : — 
In an acid of 
» 
v 
a 
34° B, 
the passivity of 
iron 
ceased after 11 days. 
32° B, 
yy 
yy 
5 „ 
30° B, 
yy 
yy 
32 hours. 
28° B, 
yy 
yy 
26 „ 
25° B, 
yy 
yy 
24 „ 
20* B, 
» 
V 
12 „ 
