Because I'm not totally sure this is a reliable account. And it's rather avague of specific details, other than some dodgy sounding phorensic tests.
Among the criminal abuses of the diffusion of knowledge which characterises the present times, the administration of opium, or its tincture, concealed in various vehicles, by the lower orders, with the most felonious purposes, holds a conspicuous place. An atrocious crime of this nature, says Mr. Lawrence in his Lectures, was brought specially under my notice, about a year ago, in examining, by desire of the magistrates of Glasgow, the contents of the stomach of a man who had fallen a victim to these murderous devices. Here the laudanum had been largely mixed with strong beer, and was sensible to the smell, in the liquor extracted by the stomach pump. One portion of that liquor, treated with acetate of lead, afforded an insoluble precipitate, from which an acid, strongly reddening permuriate of iron, was separated by the agency of the sulphuric. Another portion afforded directly, with a few drops of the permuriate of iron, an evident reddish brown tinge, very different from the drab or fawn coloured precipitate occasioned in strong beer of the same quality by the same salt of iron. Other experiments were made, which it is unnecessary to detail at present. The chemical facts, joined to a body of circumstantial evidence, led to a conviction of the guilty pair, a man and wife, who were accordingly executed.
When opium is dissolved in porter (good London), the detection of the drug becomes much more difficult than when it is dissolved in strong beer; for permuriate of iron produces with porter (lightened with an equal volume of water) nearly the same brownish colour, whether it be used as delivered by the brewer, or mixed with laudanum to the extent of thirty drops in two ounce measures. A very copious grey coloured precipitate is thrown down from London brown stout by solution of acetate of lead - nearly as copious, in fact, as from porter drugged, as above, with tincture of opium. And when these two precipitates, washed in filters are decomposed by a little dilute sulphuric acid, they afford two liquids, which strike nearly the same red brown tints with permuriate of iron. It is difficult to resist the evidence thus disclosed or the presence of opium in genuine London porter. Tincture of hop, diffused through water, becomes, with a few drops of permuriare of iron, a greenish liquid, quite different from the diluted porter treated in the same way. Porter becomes turbid when super-saturated with water of ammonia, and lets fall a brown sediment, which, collected and washed on a filter, bears some resemblance to impure morphia, but possesses a very remarkable peculiarity: it neither reddens with nitric acid, nor does it suffer morphia mixed with it to be thereby reddened, or at least the redness is merely momentary, and passes on the slightest heat into a light yellow shade. This precipitate I shall make the subject of future researches. Tincture of hops, which becomes slightly turbid on mixing with water, is rendered limpid by super-saturation with ammonia. It might be imagined that bone black (animal charcoal) would decolour porter, so that the agency of permuriate of iron on its supposed meconic acid might be made more manifest; but this process is at best fallacious; since bone-black boiled with a portion of dilute solution of opium, deprives it almost entirely of the power of affecting permuriate of iron ; while the corresponding portion receives from that salt a deep red brown colour."
Freeman's Journal - Wednesday 28 April 1830, page 4.
The main thrust seems to be: it's hard to detect opium in Porter. Rather than: I found opium in lods of samples of Porter.
I'm still not convinced about opium adulteration.