In carlicr !~ublicutions~~2 from this laboratory a new sclic~nc of qualitative analysis based on the decomposition of thiosalts was put forward. In this scheme the original mixture WiLS cvaporatcd to dryness wit11 conccntratcd nitric and hydrochloric acicls succcssivcly, the rcsicluc was taken II!> wit!1 clilutc hydrochloric acid and the resulting mixture oxiclizccl with bromine water and filtcrcc!. The filtrate containing the soluble salts was trcatccl with allcali sulphiclo rcagcnt for tllc dctcction of basic raclicnls and the unclissolvccl rcsicluc was tc3tccl for insoluble substances.
No systematic scllcmc for tllc detection of insoluble substances has yet been dcvisccl. Most textbooks on qualitative analysis give mcrcly a list of com!>ounds which arc tcrmcd "insoluble su!>stanccs" and a certain nurnbcr of gcncral tests which may bc applied one after unothcr to dctcct thcsc substances in tllc: insoluble residue. Tlu2rc is not only no clefinitc schcn~c for the iclcntification of t!lc so-called insoluble substances but there is also no ngrccmcnt among different authors with regard to the inclusion of cliffcrent compouncls in the list of insoluble substances. Many compounds arc classed as insoluble substances by some autllors and not by others. This must naturally bc confusing to the ordinary student, who has not only no dcfinitc scheme to guiclc him in tile analysis of the insoluble residue, but does not cvcn know wliicll compounds to look for in sucli a rcsiduc.
Wl~cn one: consiclcrs how thoroughly the methods for the identification of acids and bases in clualitativc analysis liavc been systcmatizcd thcrc does npt appear to bc any reason why a similar scllcmc shoulcl not also bc available for the systematic dctcction of insoluble substances. Wit11 such a schcmc the student would no doubt bc able to tackle this problctn with more conficlencc and less chance of error. l3cforc sucll a scheme can bc devised it is necessary to have a clear understanding of the bchaviour of substances whicll may bc considcrcd to be insoluble. Thcrc is considerable uncertainty in t.hc mind of tile ordinary student chiefly because of the discrepancies on this point in the current textbooks of qualitative analysis. Many substances such as fused lead chromate and mercuric sulphidc, which arc sometimes classed as insoluble substances dissolve easily, provided the mixture is evaporated to dryness successively with concentrated nitric and hydrochloric acids, and there is no reason why they should be classed as insoluble substances. On the other hand there arc many substances such as the ignited phosphates of titanium, thorium, References p. IO VOL.