I'm not sure what you mean, but weighing by difference is when you:
The result is what is called the "Tare" weight. That is the weight of the contents ONLY .
And that would not be the mass of the substance,,,it would be the weight. Mass is a determination of how much space is taken up by the substance.
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Put the dog onto a scale. If it won't stay on the scale, then try weighing yourself first, then weighing yourself while holding the dog, then take the difference in weight.
It doesn't make any difference what substance you're weighing, the value of 1/2 an ounce in grams will always be 14.1875 !
Philipp von Jolly invented the modern weighing scale in 1874.
once upon of the time
Weighing scale is called "秤" (chéng) in Chinese.