Amrad | 21/2 | + 57.6 Australian, 1 | 5 | — 28.2 Australian, 2 | 5 | — 26.4 Brazilian, 1 | 21/2 | — 36.8 Brazilian, 2 | 21/2 | + 21.0 Dextrin, 1 | 5 | +148.0 Dextrin, 2 | 5 | +133.2 Ghatti, 1 | 5 | — 39.2 Ghatti, 2 | 5 | — 80.4 ----------------+----------------+-----------------

These numbers do not show any marked connection between the viscosity, etc., of a gum and its specific rotatory power.

When gum arabic solution is treated with alcohol the gum is precipitated entirely if a large excess of spirit be used.  With a view to seeing if the precipitate yielded by the partial precipitation of a gum solution was identical in properties to the original gum, we examined several such precipitates from various gums to ascertain their rotatory power.  We found in each case that the specific rotatory power of the alcohol precipitate redissolved in water was not the same as that of the original gum.  In other words these gums contained at least two bodies of different rotatory powers, of which one is more soluble in alcohol than the other.  O’Sullivan obtained similar results with pure arabin.  The experiments were conducted in the following manner: 

(a.) Five grammes of a dextro-rotatory gum (No. 3 in table) were dissolved in 20 c.c. of water.  To the solution was added 90 c.c. of 95 per cent. alcohol.  The white precipitate which formed was thrown on to a tared filter and washed with 30 c.c. more alcohol.  The total filtrate therefore was 140 c.c.  The precipitate was dried and weighed = 2.794 grammes or 55.88 per cent. of the total gum.  The precipitate was then redissolved in water, bleached as before and diluted to a 5 per cent. solution.  This was then examined in the polarimeter.  Readings gave the value [alpha]_{D} = +58.4 deg..  The previous rotatory power of the gum was +66 deg..  Now the alcohol was driven off from the filtrate, which, allowing for the 11.95 per cent. of water in the gum, should contain 32.17 per cent. of gum.  The alcohol-free liquid was then diluted to a known volume (for 5 per cent, solution), and [alpha]_{J} found to be +57.7 deg..  This experiment was then repeated again, using 5 grammes of No. 3, when 3.5805 grammes of precipitate were obtained, using the same volumes of alcohol and water.  The precipitate gave [alpha]_{J} = +57.4 deg.; the filtrate treated as before, only the percentage of gum dissolved being directly determined instead of being calculated by difference, gave [alpha]_{J} = +52.5 deg..

(b.) Another gum (No. 9) with [alpha]_{J} = -38.2 deg. and containing 13.86 per cent, of moisture, gave 2.3315 grms. of precipitate when similarly treated.  The precipitate gave when redissolved in water [alpha]_{J} = -20.8 deg..  The filtrate containing 39.5 per cent, real gum gave [alpha]_{J} = -67.5 deg., so that the least laevo-rotatory gum. was precipitated by the alcohol.