Complex sentence: If such a solution had been substituted for the bottle Mrs Pritchard always kept, the cyanide would kill her but the gas covered the smell of almonds that would remain for only a short time.
Split into simpler sentences: If such a solution was substituted for the bottle Mrs Pritchard always kept, the cyanide would kill her but no trace would soon remain. The gas covered the smell of almonds that would remain for only a short time.