Orange is composed of sulphide of calcium which has 
been heated with 1 or 2 per cent of binoxide of manganese. 
The lime for the preparation of the above-mentioned two 
colours was produced by the calcination of oyster shells. 
An orange -coloured phosplioi escence is also produced by 
sulphide of barium. 
Blue and Violet are each sulphides of calcium which have 
been prepared from precipitated carbonate of lime. 
Yelloivish Green is simply sulphide of strontium. 
Telloiu is sulphide of strontium which has been calcined 
with 4 or 5 per cent of sulphide of antimony. 
“ On the t}^pes of Compound Statement involving four 
Classes/’ by Professor W. K. Clifford, M.A., F.R.S. Com- 
municated by Professor W. S. Jevons, M.A., F.RS. 
Professor Stanley Jevons has enumerated^ the types of 
compound statement involving three classes, among which the 
premises of a syllogism appear as a type of four-fold statement. 
He propounded at the same time the corresponding problem 
of enumeration for four classes, which is solved in the present 
communication. The reader is referred to the paper or the 
book just mentioned for further explanation of the nature 
and purpose of the problem than is to be found in Aid. 1. 
It may however be premised that the letters A, B, C, I), 
denote four classes or terms (for example hard, wet, black, 
nice), and that according to a convenient notation of De 
Morgan’s, the small letters, a, h, c, d denote the complement- 
ary classes or contrary terms (not hard, not wet, not black, 
not nice). A simple statement is of the form ABCD=0. 
(no liard, wet, black, nice things exist, or, which is the same 
thing, all hard, wet, black things are nasty). The statement 
ABC = 0 (no hard, wet, black things exist, or all hard, black 
* Proceedings of the Manchester Phil. Soc. vol. vi. pp. 63 — 68, and 
Memoirs, Third Series, vol. r. pp, 119 — 130. 
The Principles of Science, vol. i. pp. lol — 164, 
