but *s-bʰr̥Hg- > PIE *spʰr̥Hg- > Sanskritsphūrjati.
Discussion
Siebs proposed this law in the Zeitschrift für vergleichende Sprachforschung auf dem Gebiete der indogermanischen Sprachen, as Anlautstudien.[1]Oswald Szemerényi rejected this rule, explaining that it is untenable and cites the contradiction present in Avestanzdī from PIE *s-dʰi "be!" as counterproof.[2] However, the PIE form is more accurately reconstructed as *h₁s-dʰí from *h₁es- (so not an s-mobile) and thus Siebs' law appears to demand that the sibilant and aspirated stop are both adjacent and tautosyllabic, something which is known to only occur in word-initial position in Proto-Indo-European anyway.