Another mystery solved

Here’s a short fragment of Accident by Agatha Christie.

Evans paid no attention, but went on. ‘You interrupted me just now. After Marsh’s test, Merrowdene heated a substance in a test tube, the metallic residue he dissolved in water and then precipitated it by adding silver nitrate. That was a test for chlorates. A neat, unassuming little test. But I chanced to read these words in a book that stood open on the table. “H₂SO₄ decomposes chlorates with evolution of Cl₂O₄. If heated, violent explosions occur, the mixture ought therefore to be kept cool and only very small quantities used.”’

What book was that? Googling gave me The Elements of Chemical Arithmetic with a Short System of Elementary Qualitative Analysis by J. Milnor Coit, Ph.D., published in 1886. On page 80, section 103, I’ve found the original description (shortened in Agatha Christie’s version):

H₂SO₄ decomposes chlorates with evolution of Cl₂O₄, a greenish-yellow gas having a powerful odor. If heated, violent explosions occur; the mixture ought therefore to be kept cold, and only very small quantities should be used.

The full text of this, apparently, still very useful book is copyright-free.